diff --git a/src/function/set/setUnion.js b/src/function/set/setUnion.js
index ce53bea081..77355b49e1 100644
--- a/src/function/set/setUnion.js
+++ b/src/function/set/setUnion.js
@@ -2,9 +2,9 @@ import { flatten } from '../../utils/array.js'
 import { factory } from '../../utils/factory.js'
 
 const name = 'setUnion'
-const dependencies = ['typed', 'size', 'concat', 'subset', 'setIntersect', 'setSymDifference', 'Index']
+const dependencies = ['typed', 'size', 'concat', 'subset', 'setDifference', 'setIntersect', 'Index']
 
-export const createSetUnion = /* #__PURE__ */ factory(name, dependencies, ({ typed, size, concat, subset, setIntersect, setSymDifference, Index }) => {
+export const createSetUnion = /* #__PURE__ */ factory(name, dependencies, ({ typed, size, concat, subset, setDifference, setIntersect, Index }) => {
   /**
    * Create the union of two (multi)sets.
    * Multi-dimension arrays will be converted to single-dimension arrays before the operation.
@@ -35,7 +35,9 @@ export const createSetUnion = /* #__PURE__ */ factory(name, dependencies, ({ typ
       }
       const b1 = flatten(a1)
       const b2 = flatten(a2)
-      return concat(setSymDifference(b1, b2), setIntersect(b1, b2))
+      // union = elements only in a1 + elements in both + elements only in a2
+      // (all three sub-results are sorted by setDifference/setIntersect internally)
+      return concat(concat(setDifference(b1, b2), setIntersect(b1, b2)), setDifference(b2, b1))
     }
   })
 })
diff --git a/test/unit-tests/function/set/setUnion.test.js b/test/unit-tests/function/set/setUnion.test.js
index b8bf6f5c68..b51a7bf5fb 100644
--- a/test/unit-tests/function/set/setUnion.test.js
+++ b/test/unit-tests/function/set/setUnion.test.js
@@ -9,10 +9,13 @@ describe('setUnion', function () {
     assert.deepStrictEqual(math.setUnion(['a', 'b'], ['c', 'd']), ['a', 'b', 'c', 'd'])
     assert.deepStrictEqual(math.setUnion([], [3, 4]), [3, 4])
     assert.deepStrictEqual(math.setUnion([], []), [])
+    // sets with overlapping elements must include all unique elements in sorted order
+    assert.deepStrictEqual(math.setUnion([1, 2, 3, 4], [3, 4, 5, 6]), [1, 2, 3, 4, 5, 6])
+    assert.deepStrictEqual(math.setUnion([1, 2], [2, 3]), [1, 2, 3])
   })
 
   it('should return the union of two multisets', function () {
-    assert.deepStrictEqual(math.setUnion([1, 1, 2, 3, 4, 4], [1, 2, 3, 4, 4, 4]), [1, 4, 1, 2, 3, 4, 4])
+    assert.deepStrictEqual(math.setUnion([1, 1, 2, 3, 4, 4], [1, 2, 3, 4, 4, 4]), [1, 1, 2, 3, 4, 4, 4])
   })
 
   it('should return the same type of output as the inputs', function () {