Gulbis told golf.com they are dating.
"I'll let Dustin handle our PR," Gulbis said with a laugh after confirming to the website they are dating.
Johnson declined comment.
The 25-year-old Johnson is a rising American star with two victories last year, including close calls in the U.S. Open and PGA Championship. He was the top American on the world money list in 2010. The 28-year-old Gulbis, who had a relationship with Pittsburgh Steelers quarterback Ben Roethlisberger in 2005, struggled on the LPGA Tour while recovering from back surgery.