Kristen Stewart's Mother Files for Divorce

Kristen Stewart is having a rough summer ... her mom has now filed for divorce ... TMZ has learned.

Jules Stewart filed the divorce petition on August 17 in L.A. County Superior Court. Jules, who married John Stewart 27 years ago, cites irreconcilable differences.

Jules filed the divorce docs herself, without using a lawyer.

As for the date of separation, Jules says she and her hubby have been estranged since June 15, 2010.

Both parents work in film production. Jules is a writer/director. John is a TV producer and stage manager.