diff --git a/CHANGELOG.md b/CHANGELOG.md
index 47303d0a4..e4ba2bf82 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # 0.18.14
 
+* Add support for decoding Document representations of untagged unions
 * Update aws-http4s clients using json to have a maxArity of Int.MaxValue
 
 # 0.18.13
diff --git a/modules/bootstrapped/test/src/smithy4s/DocumentSpec.scala b/modules/bootstrapped/test/src/smithy4s/DocumentSpec.scala
index 17fdcaddb..0c9f549bb 100644
--- a/modules/bootstrapped/test/src/smithy4s/DocumentSpec.scala
+++ b/modules/bootstrapped/test/src/smithy4s/DocumentSpec.scala
@@ -22,6 +22,7 @@ import smithy4s.example.IntList
 import alloy.Discriminated
 import munit._
 import smithy4s.example.OperationOutput
+import alloy.Untagged
 
 class DocumentSpec() extends FunSuite {
 
@@ -138,6 +139,30 @@ class DocumentSpec() extends FunSuite {
     expect.same(roundTripped, Right(fooOrBar))
   }
 
+  test("untagged unions encoding") {
+    implicit val eitherSchema: Schema[Either[Long, String]] = {
+      val left = Schema.long
+      val right = Schema.string
+
+      Schema
+        .either(left, right)
+        .addHints(
+          Untagged()
+        )
+    }
+    val longOrString: Either[Long, String] = Right("hello")
+
+    val document = Document.encode(longOrString)
+    import Document._
+    val expectedDocument =
+      Document.fromString("hello")
+
+    val roundTripped = Document.decode[Either[Long, String]](document)
+
+    expect.same(document, expectedDocument)
+    expect.same(roundTripped, Right(longOrString))
+  }
+
   test("discriminated unions encoding - empty structure alternative") {
     val fooOrBaz: Either[Foo, Baz] = Right(Baz())
 
diff --git a/modules/core/src/smithy4s/internals/DocumentDecoderSchemaVisitor.scala b/modules/core/src/smithy4s/internals/DocumentDecoderSchemaVisitor.scala
index abaa7e9b7..0f4fcb1dd 100644
--- a/modules/core/src/smithy4s/internals/DocumentDecoderSchemaVisitor.scala
+++ b/modules/core/src/smithy4s/internals/DocumentDecoderSchemaVisitor.scala
@@ -33,6 +33,7 @@ import smithy4s.schema._
 import java.util.Base64
 import java.util.UUID
 import scala.collection.immutable.ListMap
+import alloy.Untagged
 
 trait DocumentDecoder[A] { self =>
   def apply(history: List[PayloadPath.Segment], document: Document): A
@@ -445,6 +446,34 @@ class DocumentDecoderSchemaVisitor(
 
   }
 
+  private def untaggedUnion[S](
+      decoders: DecoderMap[S]
+  ): DocumentDecoder[S] = {
+    val decodersList = decoders.values.toList
+    val len = decodersList.length
+
+    handleUnion { (pp: List[PayloadPath.Segment], document: Document) =>
+      var z: S = null.asInstanceOf[S]
+      var i = 0
+      while (z == null && i < len) {
+        try {
+          z = decodersList(i)(pp, document)
+        } catch {
+          case _: Throwable => i += 1
+        }
+      }
+      if (z == null) {
+        throw new PayloadError(
+          PayloadPath(pp.reverse),
+          "Union",
+          "No valid alternatives found for untagged union"
+        )
+      } else {
+        z
+      }
+    }
+  }
+
   override def union[U](
       shapeId: ShapeId,
       hints: Hints,
@@ -466,6 +495,8 @@ class DocumentDecoderSchemaVisitor(
     hints match {
       case Discriminated.hint(discriminated) =>
         discriminatedUnion(discriminated, decoders)
+      case Untagged.hint(_) =>
+        untaggedUnion(decoders)
       case _ =>
         taggedUnion(decoders)
     }