He went down on one knee on a beach in Hawaii, after being inspired by a music track from the bloodthirsty series.
"There was a song in the first season, White Sandy Beach In Hawaii, which was kind of funny and ironic," says the star, who plays vampire Bill Compton in the show.
"[It] all started out as a joke and then we decided to go to Hawaii with my kids this year," continues the 39-year-old, who has two children from previous relationships – Billy, nine, and seven-year-old Lilac.
The actor has also revealed he was personally involved in designing Anna's engagement ring.
"A friend of mine is the jeweler, Kathy Waterman," Stephen told Access Hollywood. "So I went to Kathy and we did some designs."
Anna showed off the sparkler this week as the couple cosied up at a magazine party in West Hollywood. Getting affectionate at red carpet dos is a regular thing for the couple these days – but it wasn't always so.
Although they got together during filming the pilot of True Blood, they took pains to keep their romance under wraps, and were so successful that much of the cast and crew were unaware.
Carrie Preston, who plays waitress Arlene Fowler in the series, says of their relationship: "I didn't know about it until way into the season. They were very cool and professional about it."