Stevie Wonder -- Files for Divorce from Kai Morris

Stevie Wonder has just filed for divorce ... TMZ has learned.

In the divorce docs obtained by TMZ, the music icon cites irreconcilable differences. According to the papers, Wonder has been separated from wife Kai Millard Morris since October 2009.

Wonder filed the divorce papers using his legal name, Stevland Morris, and signed with two fingerprints.

In the docs, filed by disso-queen Laura Wasser, Wonder asks for joint custody of the couple's two boys, ages 10 and 7.

In the docs, Wonder agrees to pay both spousal and child support.

Although Wonder amassed his fortune long before he tied the knot with Kai back in 2001, some assets will be split ... though the papers indicate the amount has yet to be determined.