Between 1985 and 1989 Sean Penn was married to a famous singer. Who was it?

Madonna filed for divorce in December 1987 but later withdrew the papers, only to file them again in January 1989. She filed an assault complaint that she was beaten by her estranged husband, but later she had dropped it. Penn was alleged to have struck Madonna on multiple occasions, but in 2015 Madonna stated the allegations were "completely outrageous, malicious, reckless, and false”.