diff --git a/vertexai/genai/tokenizer/example_test.go b/vertexai/genai/tokenizer/example_test.go
new file mode 100644
index 000000000000..0726770f71f7
--- /dev/null
+++ b/vertexai/genai/tokenizer/example_test.go
@@ -0,0 +1,38 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package tokenizer_test
+
+import (
+	"fmt"
+	"log"
+
+	"cloud.google.com/go/vertexai/genai"
+	"cloud.google.com/go/vertexai/genai/tokenizer"
+)
+
+func ExampleTokenizer_CountTokens() {
+	tok, err := tokenizer.New("gemini-1.5-flash")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	ntoks, err := tok.CountTokens(genai.Text("a prompt"), genai.Text("another prompt"))
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println("total token count:", ntoks.TotalTokens)
+
+	// Output: total token count: 4
+}
diff --git a/vertexai/genai/tokenizer/tokenizer.go b/vertexai/genai/tokenizer/tokenizer.go
new file mode 100644
index 000000000000..b51a03db0267
--- /dev/null
+++ b/vertexai/genai/tokenizer/tokenizer.go
@@ -0,0 +1,156 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package tokenizer provides local token counting for Gemini models. This
+// tokenizer downloads its model from the web, but otherwise doesn't require
+// an API call for every CountTokens invocation.
+package tokenizer
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"cloud.google.com/go/vertexai/genai"
+	"cloud.google.com/go/vertexai/internal/sentencepiece"
+)
+
+var supportedModels = map[string]bool{
+	"gemini-1.0-pro":       true,
+	"gemini-1.5-pro":       true,
+	"gemini-1.5-flash":     true,
+	"gemini-1.0-pro-001":   true,
+	"gemini-1.0-pro-002":   true,
+	"gemini-1.5-pro-001":   true,
+	"gemini-1.5-flash-001": true,
+}
+
+// Tokenizer is a local tokenizer for text.
+type Tokenizer struct {
+	encoder *sentencepiece.Encoder
+}
+
+// CountTokensResponse is the response of [Tokenizer.CountTokens].
+type CountTokensResponse struct {
+	TotalTokens int32
+}
+
+// New creates a new [Tokenizer] from a model name; the model name is the same
+// as you would pass to a [genai.Client.GenerativeModel].
+func New(modelName string) (*Tokenizer, error) {
+	if !supportedModels[modelName] {
+		return nil, fmt.Errorf("model %s is not supported", modelName)
+	}
+
+	data, err := loadModelData(gemmaModelURL, gemmaModelHash)
+	if err != nil {
+		return nil, fmt.Errorf("loading model: %w", err)
+	}
+
+	encoder, err := sentencepiece.NewEncoder(bytes.NewReader(data))
+	if err != nil {
+		return nil, fmt.Errorf("creating encoder: %w", err)
+	}
+
+	return &Tokenizer{encoder: encoder}, nil
+}
+
+// CountTokens counts the tokens in all the given parts and returns their
+// sum. Only [genai.Text] parts are suppored; an error will be returned if
+// non-text parts are provided.
+func (tok *Tokenizer) CountTokens(parts ...genai.Part) (*CountTokensResponse, error) {
+	sum := 0
+
+	for _, part := range parts {
+		if t, ok := part.(genai.Text); ok {
+			toks := tok.encoder.Encode(string(t))
+			sum += len(toks)
+		} else {
+			return nil, fmt.Errorf("Tokenizer.CountTokens only supports Text parts")
+		}
+	}
+
+	return &CountTokensResponse{TotalTokens: int32(sum)}, nil
+}
+
+// gemmaModelURL is the URL from which we download the model file.
+const gemmaModelURL = "https://raw.githubusercontent.com/google/gemma_pytorch/33b652c465537c6158f9a472ea5700e5e770ad3f/tokenizer/tokenizer.model"
+
+// gemmaModelHash is the expected hash of the model file (as calculated
+// by [hashString]).
+const gemmaModelHash = "61a7b147390c64585d6c3543dd6fc636906c9af3865a5548f27f31aee1d4c8e2"
+
+// downloadModelFile downloads a file from the given URL.
+func downloadModelFile(url string) ([]byte, error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	return io.ReadAll(resp.Body)
+}
+
+// hashString computes a hex string of the SHA256 hash of data.
+func hashString(data []byte) string {
+	hash256 := sha256.Sum256(data)
+	return hex.EncodeToString(hash256[:])
+}
+
+// loadModelData loads model data from the given URL, using a local file-system
+// cache. wantHash is the hash (as returned by [hashString] expected on the
+// loaded data.
+//
+// Caching logic:
+//
+// Assuming $TEMP_DIR is the temporary directory used by the OS, this function
+// uses the file $TEMP_DIR/vertexai_tokenizer_model/$urlhash as a cache, where
+// $urlhash is hashString(url).
+//
+// If this cache file doesn't exist, or the data it contains doesn't match
+// wantHash, downloads data from the URL and writes it into the cache. If the
+// URL's data doesn't match the hash, an error is returned.
+func loadModelData(url string, wantHash string) ([]byte, error) {
+	urlhash := hashString([]byte(url))
+	cacheDir := filepath.Join(os.TempDir(), "vertexai_tokenizer_model")
+	cachePath := filepath.Join(cacheDir, urlhash)
+
+	cacheData, err := os.ReadFile(cachePath)
+	if err != nil || hashString(cacheData) != wantHash {
+		cacheData, err = downloadModelFile(url)
+		if err != nil {
+			return nil, fmt.Errorf("loading cache and downloading model: %w", err)
+		}
+
+		if hashString(cacheData) != wantHash {
+			return nil, fmt.Errorf("downloaded model hash mismatch")
+		}
+
+		err = os.MkdirAll(cacheDir, 0770)
+		if err != nil {
+			return nil, fmt.Errorf("creating cache dir: %w", err)
+		}
+		err = os.WriteFile(cachePath, cacheData, 0660)
+		if err != nil {
+			return nil, fmt.Errorf("writing cache file: %w", err)
+		}
+	}
+
+	return cacheData, nil
+}
diff --git a/vertexai/genai/tokenizer/tokenizer_test.go b/vertexai/genai/tokenizer/tokenizer_test.go
new file mode 100644
index 000000000000..258c34fc260b
--- /dev/null
+++ b/vertexai/genai/tokenizer/tokenizer_test.go
@@ -0,0 +1,118 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package tokenizer
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"cloud.google.com/go/vertexai/genai"
+)
+
+func TestDownload(t *testing.T) {
+	b, err := downloadModelFile(gemmaModelURL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if hashString(b) != gemmaModelHash {
+		t.Errorf("gemma model hash doesn't match")
+	}
+}
+
+func TestLoadModelData(t *testing.T) {
+	// Tests that loadModelData manages to load the model properly, and download
+	// a new one as needed.
+	checkDataAndErr := func(data []byte, err error) {
+		t.Helper()
+		if err != nil {
+			t.Error(err)
+		}
+		gotHash := hashString(data)
+		if gotHash != gemmaModelHash {
+			t.Errorf("got hash=%v, want=%v", gotHash, gemmaModelHash)
+		}
+	}
+
+	data, err := loadModelData(gemmaModelURL, gemmaModelHash)
+	checkDataAndErr(data, err)
+
+	// The cache should exist now and have the right data, try again.
+	data, err = loadModelData(gemmaModelURL, gemmaModelHash)
+	checkDataAndErr(data, err)
+
+	// Overwrite cache file with wrong data, and try again.
+	cacheDir := filepath.Join(os.TempDir(), "vertexai_tokenizer_model")
+	cachePath := filepath.Join(cacheDir, hashString([]byte(gemmaModelURL)))
+	_ = os.MkdirAll(cacheDir, 0770)
+	_ = os.WriteFile(cachePath, []byte{0, 1, 2, 3}, 0660)
+	data, err = loadModelData(gemmaModelURL, gemmaModelHash)
+	checkDataAndErr(data, err)
+}
+
+func TestCreateTokenizer(t *testing.T) {
+	// Create a tokenizer successfully
+	_, err := New("gemini-1.5-flash")
+	if err != nil {
+		t.Error(err)
+	}
+
+	// Create a tokenizer with an unsupported model
+	_, err = New("gemini-0.92")
+	if err == nil {
+		t.Errorf("got no error, want error")
+	}
+}
+
+func TestCountTokens(t *testing.T) {
+	var tests = []struct {
+		parts     []genai.Part
+		wantCount int32
+	}{
+		{[]genai.Part{genai.Text("hello world")}, 2},
+		{[]genai.Part{genai.Text("<table><th></th></table>")}, 4},
+		{[]genai.Part{genai.Text("hello world"), genai.Text("<table><th></th></table>")}, 6},
+	}
+
+	tok, err := New("gemini-1.5-flash")
+	if err != nil {
+		t.Error(err)
+	}
+
+	for i, tt := range tests {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			got, err := tok.CountTokens(tt.parts...)
+			if err != nil {
+				t.Error(err)
+			}
+			if got.TotalTokens != tt.wantCount {
+				t.Errorf("got %v, want %v", got.TotalTokens, tt.wantCount)
+			}
+		})
+	}
+}
+
+func TestCountTokensNonText(t *testing.T) {
+	tok, err := New("gemini-1.5-flash")
+	if err != nil {
+		t.Error(err)
+	}
+
+	_, err = tok.CountTokens(genai.Text("foo"), genai.ImageData("format", []byte{0, 1}))
+	if err == nil {
+		t.Error("got no error, want error")
+	}
+}