The "Secrets and Lies" actor, 41, proposed to her girlfriend on Christmas, reported Us magazine.
The couple celebrated their engagement over lunch at Seaspice in Miami on December 27.
Phillippe and Slagter, a 24-year-old law school student, have been dating for four years.
The happy couple are expected to celebrate the new year at the Five Star New Year's Eve bash on Watson Island in Miami.
Phillippe was previously married to Reese Witherspoon from 1999 to 2007. Together they share kids Ava, 16, and Deacon, 12. The actor is also father to daughter Kai, four, with ex-girlfriend Alexis Knapp, whom he dated in 2010.