"Il Grifone has rediscovered one of those strikers who has written his recent history to the sound of goals," Genoa announced on their website.
The 30-year-old forward spent six months on loan at Italian champions Juventus last season, scoring twice in 13 appearances, but failed to impress manager Antonio Conte enough to seal a permanent move.
Borriello's previous spell at Genoa during the 2007-08 season yielded 19 goals in 35 games before he returned to AC Milan, the club where he began his career and that still part-owns him.
His return to Genoa was made possible following the departure of Alberto Gilardino who has joined Bologna on loan.
