It's a substantial gift from the Massapequa-born actor, a board member of the festival since 2007, who also serves as its most high-profile celebrity name.
"We are extremely appreciative of this major contribution," festival chairman Stuart Suna said in a statement. He also praised Baldwin's "support of non-profits struggling to survive in the economic climate of the times."
This year's Hamptons International Film Festival will run from Oct. 13-17.
The gift marks Baldwin's second large donation to a Hamptons arts institution. In May, he donated $250,000 to Guild Hall.