diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index 54236b3864c..c26270ee775 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -87,6 +87,7 @@
 * Fix signature generation: recursive module `do` binding leaking compiler-generated val. ([Issue #13832](https://github.com/dotnet/fsharp/issues/13832), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: literal values in attribute arguments now preserve literal identifier name. ([Issue #13810](https://github.com/dotnet/fsharp/issues/13810), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: struct types with non-comparable/non-equatable fields now include `[<NoComparison>]`/`[<NoEquality>]`. ([Issue #15339](https://github.com/dotnet/fsharp/issues/15339), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
+* Fix `MemoryMappedFileViewStream` copying from non-array-backed `ReadOnlyMemory` values by falling back to the safe `Stream.Write` path instead of the unsupported `PositionPointer` fast path. ([Issue #20263](https://github.com/dotnet/fsharp/issues/20263), [PR #20271](https://github.com/dotnet/fsharp/pull/20271))
 * Fix signature generation: backtick escaping for identifiers containing backticks. ([Issue #15389](https://github.com/dotnet/fsharp/issues/15389), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: `private` keyword placement for prefix-style type abbreviations. ([Issue #15560](https://github.com/dotnet/fsharp/issues/15560), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: missing `[<Class>]` attribute for types without visible constructors. ([Issue #16531](https://github.com/dotnet/fsharp/issues/16531), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
diff --git a/docs/release-notes/.VisualStudio/18.vNext.md b/docs/release-notes/.VisualStudio/18.vNext.md
index cffa42edc9c..7afa9ad5659 100644
--- a/docs/release-notes/.VisualStudio/18.vNext.md
+++ b/docs/release-notes/.VisualStudio/18.vNext.md
@@ -11,6 +11,7 @@
 * Improve static compilation of state machines. ([PR #19297](https://github.com/dotnet/fsharp/pull/19297))
 * Make Alt+F1 (momentary toggle) work for inlay hints. ([PR #19421](https://github.com/dotnet/fsharp/pull/19421))
 * Fix doubled F# diagnostics in tooltips. ([Issue #16360](https://github.com/dotnet/fsharp/issues/16360))
+* Fix `NotSupportedException` in the memory-mapped-file optimization when copying `ReadOnlyMemory` into `MemoryMappedFileViewStream`. ([Issue #20263](https://github.com/dotnet/fsharp/issues/20263))
 
 ### Changed
 
diff --git a/src/Compiler/Utilities/FileSystem.fs b/src/Compiler/Utilities/FileSystem.fs
index 0265454f117..5fd055b525d 100644
--- a/src/Compiler/Utilities/FileSystem.fs
+++ b/src/Compiler/Utilities/FileSystem.fs
@@ -390,9 +390,9 @@ module MemoryMappedFileExtensions =
             let length = int64 bytes.Length
 
             trymmf length (fun stream ->
-                let span = Span<byte>(stream.PositionPointer |> NativePtr.toVoidPtr, int length)
-                bytes.Span.CopyTo(span)
-                stream.Position <- stream.Position + length)
+                match MemoryMarshal.TryGetArray(bytes) with
+                | true, segment -> stream.Write(!!segment.Array, segment.Offset, segment.Count)
+                | false, _ -> stream.Write(bytes.ToArray(), 0, bytes.Length))
 
 [<RequireQualifiedAccess>]
 module internal FileSystemUtils =