diff --git a/src/libraries/System.IO.Compression/src/Resources/Strings.resx b/src/libraries/System.IO.Compression/src/Resources/Strings.resx
index bbb10afbcf342a..9c121844d89619 100644
--- a/src/libraries/System.IO.Compression/src/Resources/Strings.resx
+++ b/src/libraries/System.IO.Compression/src/Resources/Strings.resx
@@ -212,6 +212,9 @@
   <data name="EntryTooLarge" xml:space="preserve">
     <value>Entries larger than 4GB are not supported in Update mode.</value>
   </data>
+  <data name="EntryUncompressedSizeTooLargeForUpdateMode" xml:space="preserve">
+    <value>Entries with uncompressed data larger than 2GB are not supported in Update mode.</value>
+  </data>
   <data name="EOCDNotFound" xml:space="preserve">
     <value>End of Central Directory record could not be found.</value>
   </data>
diff --git a/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.Async.cs b/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.Async.cs
index a393ea76491e0f..353b1a71e7631d 100644
--- a/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.Async.cs
+++ b/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.Async.cs
@@ -115,8 +115,14 @@ private async Task<MemoryStream> GetUncompressedDataAsync(CancellationToken canc
         {
             // this means we have never opened it before
 
-            // if _uncompressedSize > int.MaxValue, it's still okay, because MemoryStream will just
-            // grow as data is copied into it
+            // MemoryStream is backed by a single byte[] and cannot grow beyond Array.MaxLength.
+            // Validate up front before attempting the (int) cast.
+            if ((ulong)_uncompressedSize > (ulong)Array.MaxLength)
+            {
+                _currentlyOpenForWrite = false;
+                throw new InvalidDataException(SR.EntryUncompressedSizeTooLargeForUpdateMode);
+            }
+
             _storedUncompressedData = new MemoryStream((int)_uncompressedSize);
 
             if (_originallyInArchive)
diff --git a/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs b/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs
index e84666144c9379..755e88abf67550 100644
--- a/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs
+++ b/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs
@@ -481,8 +481,14 @@ private MemoryStream GetUncompressedData()
             {
                 // this means we have never opened it before
 
-                // if _uncompressedSize > int.MaxValue, it's still okay, because MemoryStream will just
-                // grow as data is copied into it
+                // MemoryStream is backed by a single byte[] and cannot grow beyond Array.MaxLength.
+                // Validate up front before attempting the (int) cast.
+                if ((ulong)_uncompressedSize > (ulong)Array.MaxLength)
+                {
+                    _currentlyOpenForWrite = false;
+                    throw new InvalidDataException(SR.EntryUncompressedSizeTooLargeForUpdateMode);
+                }
+
                 _storedUncompressedData = new MemoryStream((int)_uncompressedSize);
 
                 if (_originallyInArchive)
diff --git a/src/libraries/System.IO.Compression/tests/ZipArchive/zip_InvalidParametersAndStrangeFiles.cs b/src/libraries/System.IO.Compression/tests/ZipArchive/zip_InvalidParametersAndStrangeFiles.cs
index 46d899426459e8..25aa227a0dd63f 100644
--- a/src/libraries/System.IO.Compression/tests/ZipArchive/zip_InvalidParametersAndStrangeFiles.cs
+++ b/src/libraries/System.IO.Compression/tests/ZipArchive/zip_InvalidParametersAndStrangeFiles.cs
@@ -408,6 +408,47 @@ await Assert.ThrowsAsync<InvalidDataException>(async () =>
             await DisposeZipArchive(async, archive);
         }
 
+        [Theory]
+        [MemberData(nameof(Get_Booleans_Data))]
+        public static async Task ZipArchiveEntry_OpenInUpdateMode_UncompressedSizeGreaterThanArrayMaxLength_ThrowsInvalidData(bool async)
+        {
+            // When _uncompressedSize > Array.MaxLength, the entry's uncompressed payload
+            // cannot be loaded into a MemoryStream (which is backed by a single byte[] and
+            // therefore bounded by Array.MaxLength). The entry must be rejected up front
+            // with a descriptive InvalidDataException when opened in Update mode, rather
+            // than failing later from the MemoryStream constructor with a misleading
+            // argument-out-of-range exception caused by the (int) cast wrapping negative
+            // when _uncompressedSize exceeds int.MaxValue.
+            byte[] payload = [0xCA, 0xFE, 0xBA, 0xBE, 0xDE, 0xAD, 0xBE, 0xEF];
+            MemoryStream stream = new MemoryStream();
+
+            ZipArchive archive = await CreateZipArchive(async, stream, ZipArchiveMode.Create, leaveOpen: true);
+            ZipArchiveEntry entry = archive.CreateEntry("entry.bin", CompressionLevel.NoCompression);
+            Stream entryStream = await OpenEntryStream(async, entry);
+            await entryStream.WriteAsync(payload);
+            await DisposeStream(async, entryStream);
+            await DisposeZipArchive(async, archive);
+
+            stream.Position = 0;
+            archive = await CreateZipArchive(async, stream, ZipArchiveMode.Update, leaveOpen: true);
+            entry = archive.GetEntry("entry.bin");
+
+            FieldInfo uncompressedSizeField = typeof(ZipArchiveEntry).GetField("_uncompressedSize", BindingFlags.NonPublic | BindingFlags.Instance);
+            Assert.NotNull(uncompressedSizeField);
+            uncompressedSizeField.SetValue(entry, (long)int.MaxValue + 1L);
+
+            if (async)
+            {
+                await Assert.ThrowsAsync<InvalidDataException>(() => entry.OpenAsync());
+            }
+            else
+            {
+                Assert.Throws<InvalidDataException>(() => entry.Open());
+            }
+
+            await DisposeZipArchive(async, archive);
+        }
+
         [Theory]
         [MemberData(nameof(Get_Booleans_Data))]
         public static async Task UnseekableVeryLargeArchive_DataDescriptor_Read_Zip64(bool async)