The couple were spotted arriving in Miami on Sunday for lunch at Seaspice where they ate riverside and Slagter was seen showing off her engagement ring, an onlooker tells PEOPLE.
Phillippe’s proposal to his 24-year-old law student girlfriend is said to have happened over the Christmas holidays, and the pair will be ringing in the New Year together in Miami.
Phillippe, 41, has two children Ava, 16, and Deacon, 12, with his ex-wife Reese Witherspoon and another daughter, Kai, 4, from a previous relationship with ex-girlfriend Alexis Knapp.
In April, Phillippe told Howard Stern’s SiriusXM radio show that he was “not opposed” to getting married again and gushed about his girlfriend, saying: “She’s awesome. When people see her they have these judgments because she could be a model but she’s about to graduate Stanford Law.”
Adding: “Also what’s great about it – as you get older certain issues become more important to you. I’m very involved in civil rights issues, and so to be with a woman who could actually make an impact in the legal realm – that’s a beautiful thing.”
The pair have been dating for four years and will be ringing in 2016 at the Five Star New Year s Eve event on Watson Island headlined by deejay Robin Shulz.
• Reporting by ELIZABETH LEONARD and MELODY CHIU