Ryan O'Reilly and Robert Thomas each had two assists, and Ville Husso made 26 saves for his first win since Nov. 27 for the Blues.
Daniel Sprong scored for Washington (20-7-8). Ilya Samsonov allowed four goals on 16 shots and was pulled after two periods. Zach Fucale made seven saves in relief.
"Nobody likes losing a hockey game," Capitals coach Peter Laviolette said. "It's frustrating tonight. It was kind of a game that didn't have a lot of shots, a lot of chances. The ones they got, they had some quality to them. The ones we had in the first period had quality to them, but they just didn't go. There are things we can do better defensively to tighten that up."
Sprong put Washington ahead 1-0 at 2:02 of the first period on a wrist shot from inside the right circle.
"We had some good chances in the first 10 [minutes] and then they capitalized on our mistakes," Sprong said. "We had some good looks. They just didn't go in. At the end of the day, we didn't get a good result. We've got to put this behind us and get ready for tomorrow night [at the Minnesota Wild]."
The Capitals came out strong and were outshooting the Blues 10-3 to start the game, but Husso made sure the Blues had a chance to get back into the game, including making a left toe save on the goal line of a Brett Leason chance from close range at 1:53 of the first.
"When I came to the rink, I just focused on the start and the first period," Husso said. "It was nice to get some saves right away. After that one save, I felt like I was into the game. Sometimes it's hard to come when you're a month away and you don't play for a month. Today I was lucky to make those saves and then guys scored a couple goals. It was a good hockey game for us, and a big win."
Krug tied it 1-1 at 12:10, taking a Thomas cross-ice pass and beating Samsonov from the left circle.
"I think Krug's goal really got us settled in, to be honest with you," Blues coach Craig Berube said. "Then we started to just do what we needed to do against them."