diff --git a/packages/adapters/src/adapters/Cornerstone3D/Segmentation/labelmapImagesFromBuffer.ts b/packages/adapters/src/adapters/Cornerstone3D/Segmentation/labelmapImagesFromBuffer.ts
index 09025f2a89..22d44e6e7b 100644
--- a/packages/adapters/src/adapters/Cornerstone3D/Segmentation/labelmapImagesFromBuffer.ts
+++ b/packages/adapters/src/adapters/Cornerstone3D/Segmentation/labelmapImagesFromBuffer.ts
@@ -48,6 +48,28 @@ function prepareSegMultiframeMetadata(multiframe: Record<string, unknown>) {
   if (perFrame && !Array.isArray(perFrame)) {
     multiframe.PerFrameFunctionalGroupsSequence = [perFrame];
   }
+
+  // ImageOrientationPatient can arrive as DICOM DS strings (e.g. from DICOMweb
+  // JSON metadata). Downstream orientation checks compare it against numeric
+  // source cosines with a type-strict equality (utilities.isEqual), so an
+  // in-plane SEG would otherwise be misclassified as orthogonal/out-of-plane.
+  // Normalize it to numbers on the shared and per-frame functional groups.
+  type FunctionalGroup = {
+    PlaneOrientationSequence?: { ImageOrientationPatient?: unknown };
+  };
+  const coerceIop = (group?: FunctionalGroup) => {
+    const seq = group?.PlaneOrientationSequence;
+    if (seq && Array.isArray(seq.ImageOrientationPatient)) {
+      seq.ImageOrientationPatient = seq.ImageOrientationPatient.map(Number);
+    }
+  };
+  coerceIop(multiframe.SharedFunctionalGroupsSequence as FunctionalGroup);
+  const perFrameGroups = multiframe.PerFrameFunctionalGroupsSequence as
+    | FunctionalGroup[]
+    | undefined;
+  if (Array.isArray(perFrameGroups)) {
+    perFrameGroups.forEach(coerceIop);
+  }
 }
 
 function getFrameNumberFromImageId(imageId: string): number | undefined {
diff --git a/packages/adapters/src/adapters/helpers/checkOrientation.ts b/packages/adapters/src/adapters/helpers/checkOrientation.ts
index d95fe9cde2..a2d588c0fc 100644
--- a/packages/adapters/src/adapters/helpers/checkOrientation.ts
+++ b/packages/adapters/src/adapters/helpers/checkOrientation.ts
@@ -19,10 +19,16 @@ export default function checkOrientation(
   // Check if in plane.
   const PerFrameFunctionalGroups = PerFrameFunctionalGroupsSequence[0];
 
-  const iop =
+  const iopRaw =
     sharedImageOrientationPatient ||
     PerFrameFunctionalGroups.PlaneOrientationSequence.ImageOrientationPatient;
 
+  // ImageOrientationPatient can arrive as DICOM DS strings (e.g. from DICOMweb
+  // JSON metadata) while validOrientations are numeric source cosines. isEqual
+  // is type-strict, so a string-vs-number mismatch would make an in-plane SEG
+  // look perpendicular. Coerce to numbers before comparing.
+  const iop = Array.isArray(iopRaw) ? iopRaw.map(Number) : iopRaw;
+
   const inPlane = validOrientations.some((operation) =>
     utilities.isEqual(iop, operation, tolerance)
   );
diff --git a/packages/adapters/test/checkOrientation.jest.js b/packages/adapters/test/checkOrientation.jest.js
new file mode 100644
index 0000000000..19753bd5e8
--- /dev/null
+++ b/packages/adapters/test/checkOrientation.jest.js
@@ -0,0 +1,86 @@
+import { describe, it, expect } from '@jest/globals';
+import checkOrientation from '../src/adapters/helpers/checkOrientation';
+
+// The source data is a standard axial acquisition (row cosines [1,0,0],
+// column cosines [0,1,0]). validOrientations holds numeric cosines because they
+// are derived from the source image's parsed imagePlaneModule.
+const validOrientations = [[1, 0, 0, 0, 1, 0]];
+const sourceDataDimensions = [512, 512, 1];
+const tolerance = 1e-3;
+
+const multiframeWithSharedIop = (iop) => ({
+  Rows: 512,
+  Columns: 512,
+  SharedFunctionalGroupsSequence: {
+    PlaneOrientationSequence: { ImageOrientationPatient: iop },
+  },
+  PerFrameFunctionalGroupsSequence: [{}],
+});
+
+describe('checkOrientation', () => {
+  it('classifies a co-planar SEG as Planar when ImageOrientationPatient is numeric', () => {
+    const multiframe = multiframeWithSharedIop([1, 0, 0, 0, 1, 0]);
+    expect(
+      checkOrientation(
+        multiframe,
+        validOrientations,
+        sourceDataDimensions,
+        tolerance
+      )
+    ).toBe('Planar');
+  });
+
+  it('classifies a co-planar SEG as Planar when ImageOrientationPatient is DICOM DS strings', () => {
+    // Regression: DICOMweb JSON metadata delivers DS values as strings. The
+    // orientation check compares them against numeric source cosines with a
+    // type-strict equality, which previously misclassified this perfectly
+    // in-plane SEG as orthogonal ("Segmentations orthogonal to the acquisition
+    // plane of the source data are not yet supported.").
+    const multiframe = multiframeWithSharedIop(['1', '0', '0', '0', '1', '0']);
+    expect(
+      checkOrientation(
+        multiframe,
+        validOrientations,
+        sourceDataDimensions,
+        tolerance
+      )
+    ).toBe('Planar');
+  });
+
+  it('still classifies a genuinely perpendicular SEG as Perpendicular (string IOP)', () => {
+    // Row cosines [1,0,0], column cosines [0,0,-1]: perpendicular to the axial
+    // source. Coercion must not mask real perpendicular segmentations.
+    const multiframe = multiframeWithSharedIop(['1', '0', '0', '0', '0', '-1']);
+    expect(
+      checkOrientation(
+        multiframe,
+        validOrientations,
+        sourceDataDimensions,
+        tolerance
+      )
+    ).toBe('Perpendicular');
+  });
+
+  it('falls back to the per-frame ImageOrientationPatient when no shared group is present', () => {
+    const multiframe = {
+      Rows: 512,
+      Columns: 512,
+      SharedFunctionalGroupsSequence: {},
+      PerFrameFunctionalGroupsSequence: [
+        {
+          PlaneOrientationSequence: {
+            ImageOrientationPatient: ['1', '0', '0', '0', '1', '0'],
+          },
+        },
+      ],
+    };
+    expect(
+      checkOrientation(
+        multiframe,
+        validOrientations,
+        sourceDataDimensions,
+        tolerance
+      )
+    ).toBe('Planar');
+  });
+});