Michael Phelps engaged to Nicole Johnson

Michael Phelps has taken the plunge and gotten engaged.

The 18-time Olympic gold medalist announced on Twitter he's marrying Nicole Johnson, who was Miss California in 2010 and has dated Phelps on and off the past few years.

Phelps tweeted early Sunday: "She said yes." He also posted an Instagram photo of the couple, hugging in the snow, taken by teammate Allison Schmitt.

Phelps sister, Hilary, wrote on Facebook, "CONGRATULATIONS to these two lovebirds on their engagement! Couldn't be happier for my brother and thrilled to gain a sister, who I adore and love already!"

Johnson also took to Twitter to celebrate the engagement.

It is unclear how long the two have been together. Photos of Phelps began appearing in Johnson's social media feeds last summer.

The engagement comes as Phelps is finishing a six-month suspension from USA Swimming for his second drunken-driving arrest. The suspension ends March 6, and Phelps is expected to return to competition at Mesa, Arizona, in April.

Phelps announced he was entering a six-week, in-patient program after the suspension, days after he was arrested and charged with drunken driving in his hometown of Baltimore.

According to the police statement of probable cause, officers noticed Phelps' eyes were "red, bloodshot and his speech was mush mouth." Phelps told police he had "3 or 4 drinks" and had just left the "Horseshoe" Casino. When officers asked him to walk heel to toe in a straight line, he had "difficulty with his balance. When asked to stand on one leg, he told police "that's not happening."

A later breathalyzer revealed Phelps with a blood alcohol level of .14 -- well over the state's .08 limit.

Sources tell WJZ Phelps spent hours drinking and playing table games inside a private room at the Horseshoe Casino. Casino officials would only say it would inappropriate to comment on any ongoing investigation.

Phelps has already said he's sorry for letting people down. He made a similar apology ten years ago.