diff --git a/gems/aws-sdk-s3/CHANGELOG.md b/gems/aws-sdk-s3/CHANGELOG.md
index 2385d906d8e..27be70afbca 100644
--- a/gems/aws-sdk-s3/CHANGELOG.md
+++ b/gems/aws-sdk-s3/CHANGELOG.md
@@ -1,6 +1,8 @@
 Unreleased Changes
 ------------------
 
+* Issue - S3 Encryption Client, encryptionV2 and encryptionV3, returns a decryption error for a malformed material description.
+
 1.228.1 (2026-07-23)
 ------------------
 
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV2/kms_cipher_provider.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV2/kms_cipher_provider.rb
index 65970395b8c..edeaca5aba8 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV2/kms_cipher_provider.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV2/kms_cipher_provider.rb
@@ -50,7 +50,7 @@ def encryption_cipher(options = {})
         # @return [Cipher] Given an encryption envelope, returns a
         #   decryption cipher.
         def decryption_cipher(envelope, options = {})
-          encryption_context = Json.load(envelope['x-amz-matdesc'])
+          encryption_context = extract_encryption_context(envelope['x-amz-matdesc'])
           cek_alg = envelope['x-amz-cek-alg']
 
           case envelope['x-amz-wrap-alg']
@@ -115,6 +115,19 @@ def decryption_cipher(envelope, options = {})
 
         private
 
+        # Raise a decryption error for a malformed material description. A
+        # material description must be a JSON object.
+        def extract_encryption_context(matdesc)
+          context = Json.load(matdesc) if matdesc.is_a?(String)
+          unless context.is_a?(Hash)
+            raise Errors::DecryptionError, 'Malformed material description'
+          end
+
+          context
+        rescue Aws::Json::ParseError, EncodingError
+          raise Errors::DecryptionError, 'Malformed material description'
+        end
+
         def validate_key_wrap(key_wrap_schema)
           case key_wrap_schema
           when :kms_context then 'kms+context'
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV3/kms_cipher_provider.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV3/kms_cipher_provider.rb
index 38c61240c62..f5458a26ebe 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV3/kms_cipher_provider.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV3/kms_cipher_provider.rb
@@ -54,7 +54,7 @@ def decryption_cipher(envelope, options = {})
             cek_alg = envelope['x-amz-c']
             encryption_context =
               if !envelope['x-amz-t'].nil?
-                Json.load(envelope['x-amz-t'])
+                extract_encryption_context(envelope['x-amz-t'])
               else
                 ##= ../specification/s3-encryption/data-format/content-metadata.md#v3-only
                 ##% If the mapkey x-amz-t is not present, the default Material Description value MUST be set to an empty map (`{}`).
@@ -102,6 +102,18 @@ def decryption_cipher(envelope, options = {})
 
         private
 
+        # Raise a decryption error for a malformed material description.
+        def extract_encryption_context(matdesc)
+          context = Json.load(matdesc) if matdesc.is_a?(String)
+          unless context.is_a?(Hash)
+            raise Errors::DecryptionError, 'Malformed material description'
+          end
+
+          context
+        rescue Aws::Json::ParseError, EncodingError
+          raise Errors::DecryptionError, 'Malformed material description'
+        end
+
         def validate_key_wrap(key_wrap_schema)
           case key_wrap_schema
           when :kms_context then '12'
diff --git a/gems/aws-sdk-s3/spec/encryptionV2/kms_cipher_provider_spec.rb b/gems/aws-sdk-s3/spec/encryptionV2/kms_cipher_provider_spec.rb
new file mode 100644
index 00000000000..2f896ae0929
--- /dev/null
+++ b/gems/aws-sdk-s3/spec/encryptionV2/kms_cipher_provider_spec.rb
@@ -0,0 +1,43 @@
+require_relative '../spec_helper'
+
+module Aws
+  module S3
+    module EncryptionV2
+      describe KmsCipherProvider do
+        let(:provider) do
+          KmsCipherProvider.new(
+            kms_key_id: 'kms-key-id',
+            kms_client: KMS::Client.new(stub_responses: true),
+            key_wrap_schema: :kms_context,
+            content_encryption_schema: :aes_gcm_no_padding
+          )
+        end
+
+        describe '#decryption_cipher' do
+          # A malformed material description raises DecryptionError.
+          ['=?utf-8?B?gA==?=', '=?utf-8?B?/w==?=', '=?utf-8?B?YWJj/w==?=', "abc\xFF"].each do |matdesc|
+            it "raises DecryptionError (#{matdesc.inspect})" do
+              expect do
+                provider.decryption_cipher('x-amz-matdesc' => matdesc)
+              end.to raise_error(Errors::DecryptionError, /Malformed material description/)
+            end
+          end
+
+          # Valid JSON that isn't an object, or an absent matdesc, is malformed.
+          ['123', nil].each do |matdesc|
+            it "raises DecryptionError (#{matdesc.inspect})" do
+              envelope = {
+                'x-amz-matdesc' => matdesc,
+                'x-amz-wrap-alg' => 'kms+context',
+                'x-amz-cek-alg' => 'AES/GCM/NoPadding'
+              }
+              expect do
+                provider.decryption_cipher(envelope)
+              end.to raise_error(Errors::DecryptionError, /Malformed material description/)
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/gems/aws-sdk-s3/spec/encryptionv3/kms_cipher_provider_spec.rb b/gems/aws-sdk-s3/spec/encryptionv3/kms_cipher_provider_spec.rb
new file mode 100644
index 00000000000..38bbab26735
--- /dev/null
+++ b/gems/aws-sdk-s3/spec/encryptionv3/kms_cipher_provider_spec.rb
@@ -0,0 +1,29 @@
+require_relative '../spec_helper'
+
+module Aws
+  module S3
+    module EncryptionV3
+      describe KmsCipherProvider do
+        let(:provider) do
+          KmsCipherProvider.new(
+            kms_key_id: 'kms-key-id',
+            kms_client: KMS::Client.new(stub_responses: true),
+            key_wrap_schema: :kms_context,
+            content_encryption_schema: :alg_aes_256_gcm_hkdf_sha512_commit_key
+          )
+        end
+
+        describe '#decryption_cipher' do
+          # A malformed material description raises DecryptionError.
+          ['=?utf-8?B?gA==?=', '=?utf-8?B?/w==?=', '=?utf-8?B?YWJj/w==?=', "abc\xFF", '123'].each do |matdesc|
+            it "raises DecryptionError (#{matdesc.inspect})" do
+              expect do
+                provider.decryption_cipher('x-amz-w' => '12', 'x-amz-t' => matdesc)
+              end.to raise_error(Errors::DecryptionError, /Malformed material description/)
+            end
+          end
+        end
+      end
+    end
+  end
+end