Ex-MLB Star Matt Williams' Wife Files For Divorce

World Series champion Matt Williams is set to be a single man once again ... TMZ Sports has learned his wife just filed for divorce.

Erika Monroe filed the docs in Los Angeles County on Wednesday ... over 20 years after the couple initially married.

In the documents, Monroe listed "irreconcilable differences" as the reason for the split. The docs show she also asked for spousal support from Williams, who reportedly made around $80 million in his 17-year MLB career.

We've reached out to Monroe's attorney for comment, but so far, no word back yet.

57-year-old Williams -- a five-time All-Star who hit 378 home runs in his career -- married Monroe, a former news anchor, on July 15, 2003. The two went on to cohost pregame shows for the Arizona Diamondbacks in the years after they got hitched.

The couple also had one daughter together ... although she's an adult now, so child support won't be an issue.