The 49-year-old Gravity star was visiting the talk show for the 20th time when she thanked Leno for always being so kind.
"I'd like to say something. I'm going to start crying," Bullock said, tearing up. "You've always been so kind. That's saying a lot in this business because we like to be mean."
"There's not been one time that you haven't treated me like I had something to offer. Even when the film was horrible and you knew it, you never let me see it in your eyes. When I made crazy life decisions you never questioned [them]. You were always so welcoming and every single person on your crew was that way consistently and I just felt special even when I felt very insecure. And everyone, I think, in this room and in this country, has felt that every day that you've been in their homes. I'm so grateful that I got to be a part of this."
During Wednesday's visit the duo also discussed Bullock's hairstyles over the years, and how the talk-show host always knew she was meant to be a star.
After 22 years, Leno's final Tonight Show episode will air Thursday and star his first-ever guests, Billy Crystal and Garth Brooks. Jimmy Fallon will take over the show beginning Monday, Feb. 17 as Seth Meyers replaces him on Late Night the following week.