Borriello rejoins Genoa from Roma
"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.