Italy international Zappacosta returns to Serie A following last season's temporary switch to Roma, which was interrupted by a serious knee ligament injury.
He recovered from surgery in time to make six appearances in the Italian top flight before the end of the season, taking his total appearances for the Giallorossi to eight.
The 28-year-old, signed by Antonio Conte in 2017, has been a peripheral figure at Stamford Bridge and restricted to 34 starts and a further 18 substitute appearances.
Right-back Zappacosta has won 13 caps for his country, the most recent of which came two years ago.
Genoa finished 17th in Serie A last season.