The Ghanaian international had already agreed a deal to return to the Rossoneri, having been training with Milan since September.
This afternoon, Boateng’s contract has officially been deposited with the Serie A, meaning he is now free to be involved in competitive games for the Rossinieri.
Boateng made 100 appearances in his previous spell with the Rossoneri, scoring 17 goals and helping his side to the 2011 Scudetto.
The former Portsmouth midfielder joined Schalke in 2013 for a €10m fee, but was suspended by the Bundesliga side earlier this year for poor behaviour.
His contract with the club was terminated on December 8, and he will be hoping to get his career back on track with Milan.