The actress's partner, Morty Coyle, proposed to her in front of all their party guests at the Kibitz Room in Los Angeles.
"It was a complete surprise," Sweetin tells PEOPLE. "We are just thrilled and I am in shock still!"
The couple has a 5-month-old daughter, Beatrix, who was also there for the big moment, and Sweetin says she was thrilled to be among loved ones when it happened.
"I was glad that everybody was there," she says. "People videotaped it, it was great." And the venue was decked out specifically for the occasion as well – even though Sweetin didn't know it.
"My friend decorated the whole place with little diamonds on the table and other touches," she says. Of course, those weren't the only gems involved.
Her engagement ring, "is so beautiful," says Sweetin of her 2½-carat oval sapphire, surrounded by 18 diamonds in a snowflake shape. It's "a one-of-a-kind original," adds Coyle, from Erwin Hesz jewelers.
Still, it's a bit early to put the former Full House star (who also has a two-year-old daughter, Zoie, with ex-husband Cody Herpin) on wedding watch.
"We are in no hurry as far as dates or anything like that," says Sweetin, who adds that gay marriage rights factor into their decision to wait. Adds Coyle: "We want to wait until there's equality for everybody to get married."
And if that day doesn't come anytime soon? "That's okay," says Sweetin. "We were absolutely committed before this, anyway. We're just so excited."
Adds her new fiancé of their happy news: "Just because we weren't married doesn't mean she doesn't deserve a ring."