In another tweet, the 70-year-old singer shared a photo of his fiancee and wrote "I'd like you to meet Katie. I'm lovestruck. I took this photo in London last month."
Gallery: The Rock T-Shirt Mash-Up
Diamond has not commented further on the engagement, or revealed anything else about his bride-to-be. This will be Diamond's third marriage, and his first since he split from his second wife, Marcia Murphey, in 1995.
Photo by Paul Bergen/Redferns