Fowler wrote: "Today is National Best Friend Day so I wanted to lock mine down..." He was kneeling down on the shoreline in the first photo as he asked Stokke the question. "I won", Fowler posted on Instagram.
Fowler is the No. 7 ranked golfer in the world and has four career PGA Tour wins.
Fowler and Stokke's posts had more than 400,000 combined likes as of Saturday afternoon.
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Allison is a professional pole vaulter, as well as a model for fitness brands such as Nike and Athleta.
The two have been dating publicly for about a year.
In April, Stokke caddied for Fowler at the Masters, but Fowler finished just one stroke behind victor Patrick Reed after shooting a five under on Sunday in Augusta.