The 29-year-old only left Orient earlier this month after spending six years with the London club, but has wasted no time in finding a new club, with League Two Crawley announcing the deal on Tuesday afternoon.
"I am delighted we have managed to get this over the line and to welcome Dean to Crawley," head coach Dermot Drummy told the club's official website. "He will give us so many good options. He gets goals, he also gets a lot of assists and he's a hard-working player who gives everything every time he goes onto the pitch."
Cox, who started his career with Brighton & Hove Albion, is unavailable to represent Crawley in a competitive match until January 2 after signing outside of the transfer window.