Heidi and Spencer -- The Divorce is On

Heidi Montag is officially ending things with Spencer Pratt -- the MTV starlet has just filed for divorce at a Santa Monica courthouse.



The move comes just six weeks after Heidi filed for separation ... citing irreconcilable differences.

23-year-old Montag married 26-year-old Pratt in April 2009 -- right before she underwent one of the most intense plastic surgery runs we've ever seen.

According to court documents, Montag is asking the court to terminate Pratt's right to spousal support.