Britney Spears is back in shape.

A newly slender Spears, who only gave birth to her second child in September, surprised talk show host David Letterman on Monday night’s “Late Show” by popping up behind him as he was sitting at his desk.

“Oh, my god. . . . Nice to see you. How are you? Oh, you look wonderful,” Letterman told a smiling Spears, who showed up on the CBS show wearing a very short black dress and sporting a blond bob haircut.

“See, I’m in town, and I’m such a huge fan. . . . I had to stop by,” said Spears, prompting the talk-show host to gush, “Oh, that’s very sweet of you.”

Spears then delivered her well-prepared ego-bruising zinger: “Uh, no, I wasn’t talking about you. I was talking about Will Ferrell.”

The 24-year-old singer strode off the set when Letterman directed her backstage to meet comedy star Ferrell, his guest on the show.