The former Leyton Orient hero's contract, which was due to expire in June 2019, has been terminated by mutual consent.
Cox, who was signed by Dermot Drummy, made 27 appearances for the Reds, scoring two goals.
The midfielder tweeted: "Last season wasn’t what I wished, however I would like to thank the fans for their support and wish the club all the best. Looking forward to a new season wherever that may be."
Crawley Town said on the website: "The club wishes Dean all the best for his future career and thank him for his services to Crawley Town FC."