diff --git a/cmd/transactional.go b/cmd/transactional.go
index 044485b..0f52b35 100644
--- a/cmd/transactional.go
+++ b/cmd/transactional.go
@@ -112,10 +112,12 @@ func transactionalSendRunE(cmd *cobra.Command, args []string) error {
 	email, _ := cmd.Flags().GetString("email")
 	id, _ := cmd.Flags().GetString("id")
 	dataRaw, _ := cmd.Flags().GetString("data")
+	idempotencyKey, _ := cmd.Flags().GetString("idempotency-key")
 
 	req := api.SendTransactionalRequest{
 		Email:           email,
 		TransactionalID: id,
+		IdempotencyKey:  idempotencyKey,
 	}
 
 	if cmd.Flags().Changed("add-to-audience") {
@@ -155,6 +157,7 @@ func addTransactionalSendFlags(cmd *cobra.Command) {
 	cmd.Flags().BoolP("add-to-audience", "a", false, "Create a contact if one doesn't exist")
 	cmd.Flags().String("data", "", "Data variables as a JSON object")
 	cmd.Flags().StringArrayP("attachment", "A", nil, "Path to a file to attach (repeatable)")
+	cmd.Flags().String("idempotency-key", "", "Idempotency key to prevent duplicate sends")
 	cmd.MarkFlagRequired("email")
 	cmd.MarkFlagRequired("id")
 }
diff --git a/internal/api/transactional.go b/internal/api/transactional.go
index fe66423..bd9b461 100644
--- a/internal/api/transactional.go
+++ b/internal/api/transactional.go
@@ -27,6 +27,7 @@ type SendTransactionalRequest struct {
 	AddToAudience   *bool          `json:"addToAudience,omitempty"`
 	DataVariables   map[string]any `json:"dataVariables,omitempty"`
 	Attachments     []Attachment   `json:"attachments,omitempty"`
+	IdempotencyKey  string         `json:"-"`
 }
 
 func (c *Client) SendTransactional(req SendTransactionalRequest) error {
@@ -39,6 +40,9 @@ func (c *Client) SendTransactional(req SendTransactionalRequest) error {
 	if err != nil {
 		return err
 	}
+	if req.IdempotencyKey != "" {
+		httpReq.Header.Set("Idempotency-Key", req.IdempotencyKey)
+	}
 
 	resp, err := c.do(httpReq)
 	if err != nil {
diff --git a/internal/api/transactional_test.go b/internal/api/transactional_test.go
index d3a661c..be4f831 100644
--- a/internal/api/transactional_test.go
+++ b/internal/api/transactional_test.go
@@ -285,6 +285,54 @@ func TestSendTransactional(t *testing.T) {
 	}
 }
 
+func TestSendTransactional_IdempotencyKey(t *testing.T) {
+	tests := []struct {
+		name           string
+		idempotencyKey string
+		wantHeader     string
+	}{
+		{
+			name:           "sets header when provided",
+			idempotencyKey: "my-key-123",
+			wantHeader:     "my-key-123",
+		},
+		{
+			name:           "omits header when empty",
+			idempotencyKey: "",
+			wantHeader:     "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var gotHeader string
+			var body map[string]any
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				gotHeader = r.Header.Get("Idempotency-Key")
+				b, _ := io.ReadAll(r.Body)
+				json.Unmarshal(b, &body)
+				w.WriteHeader(http.StatusOK)
+				w.Write([]byte(`{"success":true}`))
+			}))
+			defer server.Close()
+
+			client := NewClient(server.URL, "test-key")
+			client.SendTransactional(SendTransactionalRequest{
+				Email:           "a@b.com",
+				TransactionalID: "abc",
+				IdempotencyKey:  tt.idempotencyKey,
+			})
+
+			if gotHeader != tt.wantHeader {
+				t.Errorf("Idempotency-Key header = %q, want %q", gotHeader, tt.wantHeader)
+			}
+			if _, ok := body["idempotencyKey"]; ok {
+				t.Error("idempotencyKey should not appear in request body")
+			}
+		})
+	}
+}
+
 func TestSendTransactional_RequestBody(t *testing.T) {
 	addToAudience := true
 	tests := []struct {