It must have been love at first sight!

Actress and outspoken environmentalist Amy Smart is engaged to HGTV’s gorgeous Carter Oosterhouse after dating for just five months.

The couple met at a green charity event in November 2010, and hunky Oosterhouse popped the question earlier this week to the 35-year-old star.

PHOTOS: Couples We’d Like To See!

“It happened four days ago,” a source told Us Weekly, who broke the news. “He proposed in L.A. It was a very big surprise but she is thrilled and is very excited. They are a good pair and very in sync with each other.”