Kevin Costner's Wife Files For Divorce

Kevin Costner's wife is pulling the plug on their marriage ... we've learned she just filed for divorce.

The actor's wife, Christine Baumgartner, filed Monday to end their 18-year marriage. It's unclear what led to the split. Christine cited the usual "irreconcilable differences."

Kevin's rep tells TMZ, "It is with great sadness that circumstances beyond his control have transpired which have resulted in Mr. Costner having to participate in a dissolution of marriage action." The rep went on ... "We ask that his, Christine's and their children's privacy be respected as they navigate this difficult time."

The "Yellowstone" star and his estranged wife share 3 children -- ages 12, 14 and 15.

We've learned Christine is asking for joint custody of the kids. Kevin filed his response Tuesday, in which he also asks for joint custody.

Christine is not asking for spousal support. Kevin's response, filed by disso-queen Laura Wasser, says "Spousal support to be payable to petitioner pursuant to the terms of the parties' Premarital Agreement." In other words, there's a prenup.