The 47-year-old actor said on Tuesday's episode of Watch What Happens Live that the casting decision for the role of Jack Dawson in Titanic was between him and DiCaprio, 46.
DiCaprio was ultimately cast in Titanic (1997), which became the highest-grossing film of all time until Avatar surpassed it in 2010. Titanic was directed by James Cameron and co-starred Kate Winslet, Billy Zane and Kathy Bates.
On WWHL, Dorff recalled how he was coming off the film Blood and Wine when he auditioned for Titanic, while DiCaprio had recently starred in Romeo + Juliet.
"At the time, we both had movies at Fox. I had kind of a dark movie with Jack Nicholson and he had Romeo + Juliet. Mine was not tracking box office-wise the same as Romeo + Juliet," Dorff said.
Dorff said he's moved on since losing Titanic to DiCaprio.
"It was between us, but you can't really look back," the actor said. "[Titanic] was obviously a monster of a movie. There were other movies that I'd passed on that I wished I had done, but I just keep going forward."
"When you're 20 years younger, you kind of forget certain things and have to move on," he added.
Dorff named DiCaprio as his biggest competition for roles during his early career.
"When I first started, at around 16, 17 ... me and DiCaprio always went up against each other," he said. "Matt Damon was a little older than us."
After losing Titanic, Dorff went on to star in Blade, Cecil B. Demented and Public Enemies. He recently played Roland West in True Detective Season 3 and Sheriff Bill Hollister on Deputy.