The `Knocked Up` actor popped the question to long-term girlfriend Lauren Miller last week and friends say she was shocked at the proposal because she thought he`d never do it.
One pal told Life and Style magazine: "She`d started giving up hope that he would ever pop the question, but he surprised her with the proposal."
Although Seth - who began dating the writer before he found fame - is extremely happy with Lauren, he recently claimed he will never be a hit with women because he doesn`t have the looks to match his humour.
He said: "I don`t really get any more attention from the opposite sex. And it`s definitely not true what women say about just wanting a man with a sense of humour.
"What women mean is they want a guy with a sense of humour who is really handsome. If a girl had a choice between Brad Pitt or me, she`d pick Brad Pitt. And I`m a lot funnier than he is."