diff --git a/modules/unicornBidAdapter.js b/modules/unicornBidAdapter.js
index 1fd3b2afcb..d8eb77e70b 100644
--- a/modules/unicornBidAdapter.js
+++ b/modules/unicornBidAdapter.js
@@ -1,7 +1,10 @@
-import { logInfo, deepAccess, generateUUID } from '../src/utils.js';
+import { logInfo, logWarn, deepAccess, generateUUID, canAccessWindowTop, getWindowTop, getWindowSelf } from '../src/utils.js';
+import { getWinDimensions } from '../src/utils/winDimensions.js';
 import { BANNER } from '../src/mediaTypes.js';
 import { registerBidder } from '../src/adapters/bidderFactory.js';
 import { getStorageManager } from '../src/storageManager.js';
+import { getGptSlotInfoForAdUnitCode } from '../libraries/gptUtils/gptUtils.js';
+import { getBoundingBox, getViewportOffset, getViewability } from '../libraries/percentInView/percentInView.js';
 
 /**
  * @typedef {import('../src/adapters/bidderFactory.js').BidRequest} BidRequest
@@ -13,6 +16,7 @@ const UNICORN_ENDPOINT = 'https://ds.uncn.jp/pb/0/bid.json';
 const UNICORN_DEFAULT_CURRENCY = 'JPY';
 const UNICORN_PB_COOKIE_KEY = '__pb_unicorn_aud';
 const UNICORN_PB_VERSION = '1.1';
+const ADSLOT_SIGNAL_VERSION = 1; // imp.ext.unicorn schema version
 const storage = getStorageManager({ bidderCode: BIDDER_CODE });
 
 /**
@@ -37,6 +41,106 @@ export const buildRequests = (validBidRequests, bidderRequest) => {
   };
 };
 
+/**
+ * Resolve the slot's DOM element id for a bid request, in priority order:
+ *   1) explicit ortb2Imp.ext.data.divId override (established convention,
+ *      also read by adagioBidAdapter/adagioRtdProvider/contxtfulRtdProvider);
+ *   2) GPT slot mapping (getSlotElementId) — handles code !== div id;
+ *   3) the ad unit code itself (when div id === code).
+ */
+function resolveDivId(bidRequest) {
+  const override = deepAccess(bidRequest, 'ortb2Imp.ext.data.divId');
+  if (override) return override;
+  const fromGpt = getGptSlotInfoForAdUnitCode(bidRequest.adUnitCode).divId;
+  return fromGpt || bidRequest.adUnitCode;
+}
+
+/**
+ * Walk up the ancestor chain to detect a fixed/sticky wrapper. Anchor and
+ * sticky ad units are usually a `position: fixed`/`sticky` *wrapper* around a
+ * statically positioned ad div, so checking only the slot element itself
+ * misses the common case.
+ */
+function fixedOrSticky(el) {
+  const win = el.ownerDocument?.defaultView || window;
+  let node = el;
+  let fixed = false;
+  let sticky = false;
+  while (node && node.nodeType === 1) {
+    const position = win.getComputedStyle(node).position;
+    if (position === 'fixed') fixed = true;
+    else if (position === 'sticky') sticky = true;
+    node = node.parentElement;
+  }
+  return { fixed, sticky };
+}
+
+/**
+ * Measure the ad slot's on-screen position/geometry and viewability, for the
+ * imp this bid request builds. Returns null when the slot element cannot be
+ * resolved.
+ *
+ * This runs synchronously in buildRequests, when the auction is already
+ * dispatching bid requests to bidders — by that point slot elements are
+ * expected to be in the DOM, the same assumption other adapters that call
+ * `percentInView` synchronously (33across, marsmedia, oms, ...) rely on.
+ * Measuring here — rather than pre-auction in a Real-Time Data submodule —
+ * keeps the signal scoped to this adapter's own OpenRTB payload: nothing is
+ * written back to `ortb2Imp`, so no other bidder or PBS can be affected by it.
+ */
+function measureAdslot(bidRequest) {
+  const divId = resolveDivId(bidRequest);
+  const el = divId && document.getElementById(divId);
+  if (!el) {
+    logWarn(`[UNICORN] adslot element not found for adUnit "${bidRequest.adUnitCode}" (divId="${divId}")`);
+    return null;
+  }
+
+  const size = { w: deepAccess(bidRequest, 'sizes.0.0'), h: deepAccess(bidRequest, 'sizes.0.1') };
+  const win = el.ownerDocument.defaultView;
+
+  // getBoundingBox uses Prebid's shared (per-auction, cached) getBoundingClientRect
+  // helper and applies the size override when the element measures 0x0 (e.g. an
+  // empty GPT slot div before the creative renders).
+  const box = getBoundingBox(el, size);
+
+  // offset between this window's viewport and the top window's, for slots
+  // measured from inside a friendly iframe.
+  const offset = getViewportOffset(win);
+  const dims = getWinDimensions();
+  const scrollX = dims.document.documentElement.scrollLeft || dims.document.body.scrollLeft || 0;
+  const scrollY = dims.document.documentElement.scrollTop || dims.document.body.scrollTop || 0;
+  const x = Math.round(box.left + offset.x + scrollX);
+  const y = Math.round(box.top + offset.y + scrollY);
+
+  // "Above the fold" is a document-relative property — whether the slot
+  // falls within the page's *initial* viewport — so compare the document-relative
+  // y against the viewport height, not the (scroll-dependent) viewport-relative
+  // rect.top. This keeps `pos` stable across refresh auctions after the user
+  // has scrolled.
+  const vh = dims.document.documentElement.clientHeight;
+  const pos = y < vh ? 1 : 3; // OpenRTB AdPosition: 1 = above the fold, 3 = below the fold
+
+  const topWin = canAccessWindowTop() ? getWindowTop() : getWindowSelf();
+  const ratio = Number((getViewability(el, topWin, size) / 100).toFixed(2));
+
+  const { fixed, sticky } = fixedOrSticky(el);
+
+  return {
+    pos,
+    signal: {
+      ver: ADSLOT_SIGNAL_VERSION,
+      ratio,
+      fixed,
+      sticky,
+      w: Math.round(box.width),
+      h: Math.round(box.height),
+      x,
+      y
+    }
+  };
+}
+
 /**
  * Transform BidRequest to OpenRTB-formatted BidRequest Object
  * @param {Array<BidRequest>} validBidRequests
@@ -47,17 +151,33 @@ function buildOpenRtbBidRequestPayload(validBidRequests, bidderRequest) {
   logInfo('[UNICORN] buildOpenRtbBidRequestPayload.validBidRequests:', validBidRequests);
   logInfo('[UNICORN] buildOpenRtbBidRequestPayload.bidderRequest:', bidderRequest);
   const imp = validBidRequests.map(br => {
-    return {
+    const banner = {
+      format: makeFormat(br.sizes),
+      w: br.sizes[0][0],
+      h: br.sizes[0][1]
+    };
+    const adslot = measureAdslot(br);
+    // A publisher-declared pos (via global FPD) takes priority over our own
+    // measurement.
+    const declaredPos = deepAccess(br, 'ortb2Imp.banner.pos');
+    if (declaredPos != null) {
+      banner.pos = declaredPos;
+    } else if (adslot) {
+      banner.pos = adslot.pos;
+    }
+    const impObj = {
       id: br.bidId,
-      banner: {
-        format: makeFormat(br.sizes),
-        w: br.sizes[0][0],
-        h: br.sizes[0][1]
-      },
+      banner,
       tagid: deepAccess(br, 'params.placementId') || br.adUnitCode,
       secure: 1,
       bidfloor: parseFloat(0)
     };
+    if (adslot) {
+      // Slot geometry/viewability, sent only in this adapter's own OpenRTB
+      // payload — not shared FPD, so no other bidder or PBS ever sees it.
+      impObj.ext = { unicorn: adslot.signal };
+    }
+    return impObj;
   });
   const request = {
     id: bidderRequest.bidderRequestId,
diff --git a/modules/unicornBidAdapter.md b/modules/unicornBidAdapter.md
index 52c11aae3e..6383f370a7 100644
--- a/modules/unicornBidAdapter.md
+++ b/modules/unicornBidAdapter.md
@@ -8,6 +8,22 @@
 
 Module that connects to UNICORN.
 
+For each bid request, the adapter measures the ad slot's on-screen
+position/geometry and viewability and sends it in the OpenRTB payload it
+builds — this is scoped to this adapter's own request only, nothing is
+written back to shared First Party Data:
+
+- `imp.banner.pos` — OpenRTB AdPosition (1 = above the fold, 3 = below the
+  fold). A publisher-declared `ortb2Imp.banner.pos`, if present, is used
+  instead of the measured value.
+- `imp.ext.unicorn` — `{ ver, ratio, fixed, sticky, w, h, x, y }`, where
+  `ratio` is the visible-area ratio (0–1), `fixed`/`sticky` report a
+  fixed/sticky ancestor, and `x`/`y`/`w`/`h` are the slot's document-relative
+  position and rendered size in CSS pixels.
+
+Slot element resolution order: `ortb2Imp.ext.data.divId` → GPT
+`getSlotElementId()` → the ad unit code.
+
 # Test Parameters
 
 ```js
diff --git a/test/spec/modules/unicornBidAdapter_spec.js b/test/spec/modules/unicornBidAdapter_spec.js
index 01dd298a8f..7cf7cdd81f 100644
--- a/test/spec/modules/unicornBidAdapter_spec.js
+++ b/test/spec/modules/unicornBidAdapter_spec.js
@@ -3,6 +3,9 @@ import * as utils from 'src/utils.js';
 import { spec } from 'modules/unicornBidAdapter.js';
 import 'lodash';
 import { getGlobal } from '../../../src/prebidGlobal.js';
+import * as percentInViewLib from 'libraries/percentInView/percentInView.js';
+import * as winDimensionsLib from 'src/utils/winDimensions.js';
+import { clearSlotInfoCache } from 'libraries/gptUtils/gptUtils.js';
 
 const bidRequests = [
   {
@@ -567,4 +570,177 @@ describe('unicornBidAdapterTest', () => {
       assert.deepStrictEqual(bids, []);
     });
   });
+
+  describe('adslot measurement', () => {
+    const VH = 800;
+    const createdEls = [];
+    let sandbox;
+    let origGoogletag;
+    let winDimensionsStub;
+
+    function makeSlotEl(id, rect, { position, parent } = {}) {
+      const el = document.createElement('div');
+      el.id = id;
+      if (position) el.style.position = position;
+      (parent || document.body).appendChild(el);
+      el.getBoundingClientRect = () => ({
+        top: rect.top,
+        left: rect.left,
+        right: rect.left + rect.width,
+        bottom: rect.top + rect.height,
+        width: rect.width,
+        height: rect.height
+      });
+      createdEls.push(el);
+      return el;
+    }
+
+    function bidReq(adUnitCode, overrides = {}) {
+      return Object.assign({
+        bidder: 'unicorn',
+        params: { accountId: 12345 },
+        mediaTypes: { banner: { sizes: [[300, 250]] } },
+        adUnitCode,
+        sizes: [[300, 250]],
+        bidId: 'bid-adslot',
+        bidderRequestId: 'bidder-req-adslot',
+        transactionId: 'tx-adslot',
+        auctionId: 'auction-adslot',
+        src: 'client'
+      }, overrides);
+    }
+
+    const bReq = {
+      bidderCode: 'unicorn',
+      auctionId: 'auction-adslot',
+      bidderRequestId: 'bidder-req-adslot',
+      refererInfo: {
+        ref: 'https://uni-corn.net/',
+        reachedTop: true,
+        numIframes: 0,
+        stack: ['https://uni-corn.net/']
+      }
+    };
+
+    function buildImp(br) {
+      const req = spec.buildRequests([br], bReq);
+      return JSON.parse(req.data).imp[0];
+    }
+
+    beforeEach(() => {
+      sandbox = sinon.createSandbox();
+      sandbox.stub(percentInViewLib, 'getViewportOffset').returns({ x: 0, y: 0 });
+      winDimensionsStub = sandbox.stub(winDimensionsLib, 'getWinDimensions').returns({
+        document: {
+          documentElement: { scrollLeft: 0, scrollTop: 0, clientHeight: VH },
+          body: { scrollLeft: 0, scrollTop: 0 }
+        }
+      });
+      origGoogletag = window.googletag;
+    });
+
+    afterEach(() => {
+      sandbox.restore();
+      window.googletag = origGoogletag;
+      clearSlotInfoCache();
+      createdEls.splice(0).forEach(el => el.remove());
+    });
+
+    it('adds imp.ext.unicorn and imp.banner.pos for a resolvable, above-the-fold slot', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(75);
+      makeSlotEl('adslot-1', { top: 100, left: 10, width: 300, height: 250 });
+      const imp = buildImp(bidReq('adslot-1'));
+
+      expect(imp.ext.unicorn).to.deep.equal({
+        ver: 1, ratio: 0.75, fixed: false, sticky: false, w: 300, h: 250, x: 10, y: 100
+      });
+      expect(imp.banner.pos).to.equal(1);
+    });
+
+    it('uses document-relative y, not viewport-relative rect.top, for the fold check', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(0);
+      // rect.top alone looks "above the fold", but a large scroll offset
+      // means the slot's real page position is well past one viewport height.
+      winDimensionsStub.returns({
+        document: {
+          documentElement: { scrollLeft: 0, scrollTop: 5000, clientHeight: VH },
+          body: { scrollLeft: 0, scrollTop: 0 }
+        }
+      });
+      makeSlotEl('adslot-2', { top: 50, left: 0, width: 300, height: 250 });
+      const imp = buildImp(bidReq('adslot-2'));
+
+      expect(imp.ext.unicorn.y).to.equal(5050);
+      expect(imp.banner.pos).to.equal(3);
+    });
+
+    it('applies the ad unit size override when the slot measures 0x0 (unrendered GPT slot)', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(40);
+      makeSlotEl('adslot-3', { top: 0, left: 0, width: 0, height: 0 });
+      const imp = buildImp(bidReq('adslot-3'));
+
+      expect(imp.ext.unicorn.w).to.equal(300);
+      expect(imp.ext.unicorn.h).to.equal(250);
+      expect(imp.ext.unicorn.ratio).to.equal(0.4);
+    });
+
+    it('detects a fixed ancestor wrapper, not only the slot element itself', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(100);
+      const wrapper = document.createElement('div');
+      wrapper.style.position = 'fixed';
+      document.body.appendChild(wrapper);
+      createdEls.push(wrapper);
+      makeSlotEl('adslot-4', { top: 0, left: 0, width: 300, height: 250 }, { parent: wrapper });
+      const imp = buildImp(bidReq('adslot-4'));
+
+      expect(imp.ext.unicorn.fixed).to.equal(true);
+      expect(imp.ext.unicorn.sticky).to.equal(false);
+    });
+
+    it('keeps fixed and sticky as distinct flags', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(100);
+      const wrapper = document.createElement('div');
+      wrapper.style.position = 'sticky';
+      document.body.appendChild(wrapper);
+      createdEls.push(wrapper);
+      makeSlotEl('adslot-5', { top: 0, left: 0, width: 300, height: 250 }, { parent: wrapper });
+      const imp = buildImp(bidReq('adslot-5'));
+
+      expect(imp.ext.unicorn.sticky).to.equal(true);
+      expect(imp.ext.unicorn.fixed).to.equal(false);
+    });
+
+    it('prefers a publisher-declared ortb2Imp.banner.pos over the measured value', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(0);
+      makeSlotEl('adslot-6', { top: 5000, left: 0, width: 300, height: 250 });
+      const br = bidReq('adslot-6', { ortb2Imp: { banner: { pos: 7 } } });
+      const imp = buildImp(br);
+
+      expect(imp.banner.pos).to.equal(7);
+    });
+
+    it('resolves the slot element via GPT slot mapping when adUnitCode differs from the div id', () => {
+      sandbox.stub(percentInViewLib, 'getViewability').returns(50);
+      makeSlotEl('gpt-mapped-div', { top: 0, left: 0, width: 300, height: 250 });
+      window.googletag = {
+        pubads: () => ({
+          getSlots: () => [{
+            getAdUnitPath: () => '/1234/gpt-ad-unit-code',
+            getSlotElementId: () => 'gpt-mapped-div'
+          }]
+        })
+      };
+      const imp = buildImp(bidReq('/1234/gpt-ad-unit-code'));
+
+      expect(imp.ext.unicorn).to.not.equal(undefined);
+      expect(imp.ext.unicorn.w).to.equal(300);
+    });
+
+    it('omits ext.unicorn and banner.pos when the slot element cannot be resolved', () => {
+      const imp = buildImp(bidReq('adslot-does-not-exist'));
+
+      expect(imp.ext).to.equal(undefined);
+      expect(imp.banner.pos).to.equal(undefined);
+    });
+  });
 });