The 20-foot tall, 180-foot wide LED screen will display the word "casino" from the 26-story hotel tower and can also be used to play video. It's expected to be completed later this month.
Officials say the sign will face the Canadian side of Niagara Falls and is intended to let tourists know what the largely unmarked tower is.
Canadian casinos just across the border are more clearly labeled.
The Senecas also operate casinos in Buffalo and Salamanca.