The newly married Osbournes star, whose wedding album appears in next week's HELLO! Magazine, rushed to the lady's aid on the Kona-Kohala coast of West Hawaii.
Jack and his friend Tyler pulled the woman from the sea after she had suffered a heart attack.
The 26-year-old impressed his new wife Lisa Stelly with his heroic turn.
She tweeted: "Really proud of Jack and his friend Tyler who saved a woman on the beach yesterday who'd had a heart attack and drown (sic).
"They resuscitated her until the ambulance came.
"We just found out she's in a stable condition! Awesome!"
Jack married actress and model Lisa during a fun-filled and emotional ceremony on Sunday.
HELLO! was there to witness the happy couple exchange their vows in front of a 48-strong congregation of immediate family and lifelong friends.
Those present included Jack's parents Sharon and Ozzy (who wed in neighbouring Maui 30 years ago) and sister Kelly who was one of the bridesmaids.
It's been an eventful few months for Jack.
As well as tying the knot, he welcomed his first daughter, Pearl, with Lisa in April.
Then just three weeks later, there was shock for the family when he was diagnosed with multiple sclerosis.
Jack told HELLO! at the time: "While I was waiting for the final results, I got really, really angry.
Then I got really sad for about two days, and after that I realised: 'Being angry and upset is not going to do anything at this point, if anything it's only going to make it worse…
"'adapt and overcome' is my new motto."
JACK AND LISA'S EXCLUSIVE WEDDING ALBUM AND INTERVIEW WILL APPEAR IN NEXT WEEK'S ISSUE OF HELLO! ON SALE MONDAY 15th OCTOBER