The former AC Milan, Inter Milan and Real Madrid forward had been without a club since a brief pre-season training stint with Verona in July 2017, and last played an official league game in May 2016 for Sampdoria.
Cassano, who won 39 caps for Italy, had agreed to train with Entella without signing a contract, but the 36-year-old has now revealed his decision to call it quits.
In a statement released via reporter Pierluigi Pardo, Cassano said: "Over the last few days of training, I realised that I no longer have the mentality to train consistently.
"In order to play football, you need passion and talent, but above all determination and at this moment I have other priorities.
"Now the second half of my life begins. I am curious and fired up to prove first of all to myself that I can do good things even without the help of my feet."