diff --git a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
index 8963a0c180..a460135c22 100644
--- a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
+++ b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
@@ -1044,6 +1044,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 ( "o4-mini-", ModelEncoding.O200kBase ),  // e.g. o4-mini
 
                                                                 // chat
+                                                                ( "gpt-5.4-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.3-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.2-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.1-", ModelEncoding.O200kBase ),
@@ -1074,6 +1075,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 { "o4-mini", ModelEncoding.O200kBase },
 
                                                                 // chat
+                                                                { "gpt-5.4", ModelEncoding.O200kBase },
                                                                 { "gpt-5.3", ModelEncoding.O200kBase },
                                                                 { "gpt-5.2", ModelEncoding.O200kBase },
                                                                 { "gpt-5.1", ModelEncoding.O200kBase },
diff --git a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
index 059c6e6035..cc3814143b 100644
--- a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
+++ b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
@@ -39,6 +39,7 @@ public class TiktokenTests
         public static Tokenizer GPT5_1 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.1");
         public static Tokenizer GPT5_2 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.2");
         public static Tokenizer GPT5_3 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.3");
+        public static Tokenizer GPT5_4 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.4");
         public static Tokenizer Phi4 { get; } = TiktokenTokenizer.CreateForModel("phi-4");
         public static TiktokenTokenizer GptOss { get; } = TiktokenTokenizer.CreateForModel("gpt-oss-20b");
 
@@ -424,6 +425,8 @@ public void TestEncodeR50kBase()
         [InlineData("gpt-5.2-mini")]
         [InlineData("gpt-5.3")]
         [InlineData("gpt-5.3-mini")]
+        [InlineData("gpt-5.4")]
+        [InlineData("gpt-5.4-nano")]
         [InlineData("chatgpt-4o-")]
         [InlineData("gpt-4")]
         [InlineData("gpt-4-")]
@@ -545,6 +548,7 @@ public void TestEncodingNamesNegativeCases()
         [InlineData("gpt-5.1")]
         [InlineData("gpt-5.2")]
         [InlineData("gpt-5.3")]
+        [InlineData("gpt-5.4")]
         [InlineData("o1")]
         [InlineData("o3")]
         [InlineData("o4-mini")]
@@ -924,4 +928,3 @@ public void TestLargeInputConsistency(int length)
         }
     }
 }
-