Millwall made it two wins out of two under new boss Neil Harris as Zian Flemming's early free-kick was enough for three points against struggling Watford.
Flemming beat keeper Ben Hamer from 30 yards as Valerien Ismael's Hornets suffered their fifth defeat in six games, a run which looks to have put them out of contention for a play-off place.
Head coach Ismael had demanded more from his players following last week's home loss to Huddersfield, but despite enjoying much of the possession, they gave Lions keeper Matija Sarkic few anxious moments.
The win lifted Millwall up one place to 19th in the table, with Watford dropping one to 12th, now 12 points adrift of the top six following a run of five defeats in six games.