diff --git a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
index 28e272e267..54d4daa9e3 100644
--- a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
+++ b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
@@ -1026,6 +1026,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                             [
                                                                 // chat
                                                                 ( "o1-", ModelEncoding.O200kBase ),       // e.g. o1-mini
+                                                                ( "o3-", ModelEncoding.O200kBase ),       // e.g. o3-mini
                                                                 ( "gpt-4o-", ModelEncoding.O200kBase),    // e.g., gpt-4o-2024-05-13
                                                                 ( "gpt-4-", ModelEncoding.Cl100kBase),    // e.g., gpt-4-0314, etc., plus gpt-4-32k
                                                                 ( "gpt-3.5-", ModelEncoding.Cl100kBase),  // e.g, gpt-3.5-turbo-0301, -0401, etc.
@@ -1038,6 +1039,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 // chat
                                                                 { "gpt-4o", ModelEncoding.O200kBase },
                                                                 { "o1", ModelEncoding.O200kBase },
+                                                                { "o3", ModelEncoding.O200kBase },
                                                                 { "gpt-4", ModelEncoding.Cl100kBase },
                                                                 { "gpt-3.5-turbo", ModelEncoding.Cl100kBase },
                                                                 { "gpt-3.5-turbo-16k", ModelEncoding.Cl100kBase },
diff --git a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
index 1e7cad6890..6333723e7d 100644
--- a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
+++ b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
@@ -394,6 +394,10 @@ public void TestEncodeR50kBase()
         [Theory]
         [InlineData("o1")]
         [InlineData("o1-")]
+        [InlineData("o1-mini")]
+        [InlineData("o3")]
+        [InlineData("o3-")]
+        [InlineData("o3-mini")]
         [InlineData("gpt-4o")]
         [InlineData("gpt-4o-")]
         [InlineData("gpt-4")]
@@ -496,6 +500,7 @@ public void TestEncodingNamesNegativeCases()
         [InlineData("gpt-4")]
         [InlineData("gpt-4o")]
         [InlineData("o1")]
+        [InlineData("o3")]
         [InlineData("text-davinci-003")]
         [InlineData("text-curie-001")]
         [InlineData("text-davinci-edit-001")]