Thomas, who made 39 saves overall, turned aside shot after shot in the final period as the Caps struggled to break a seven-game losing streak. But in the end, he allowed only a single third-period tally, to Karl Alzner, which brought Washington to within one.
The Bruins got a goal and an assist each from Patrice Bergeron and Blake Wheeler. Andrew Ference added a first-period goal as Boston snapped a three-game losing streak.
Michal Neuvirth turned aside 18-of-21 shots, but it wasn't enough to stop the Capitals' slide, which reached eight straight games with the latest setback.