Kevin-Prince Boateng had been training with AC Milan since September after his Schalke contract was terminated last month.
AC Milan has re-signed midfielder Kevin-Prince Boateng on a free transfer.
Boateng's return the San Siro - believed to be on a six-month contract until the end of the season - was confirmed via the Serie A on Monday.
The 28-year-old German-born Ghanaian, who first represented Milan between 2010 and 2013, had been training with Sinisa Mihajlovic's men with a view to a deal since September, though his Schalke contract was not terminated until last month.
He did not make a single appearance for Schalke in 2015-16, having been suspended following May's 2-0 Bundesliga defeat to Cologne.