First-half goals from Lee Gregory and Aiden O’Brien saw
Gregory opened the scoring by firing in from O’Brien’s cross and the winger made it 2-0 when he shot past Lawrence Vigouroux from inside the box.
Steve Morison should have made the victory more comfortable, with Vigouroux denying him three times.
The Robins, who are 20th in the table, have failed to win for seven matches.
Millwall manager Neil Harris told BBC Radio London:
I’m absolutely delighted, firstly with the clean sheet because we worked hard the last 10 days.
“I’m delighted with our attacking play, the amount of crosses we got. I’m delighted with the win.
“We’re edging back to where we want to be with home form, but what’s important now is that we recover and prepare correctly for what’s a big game for us on Tuesday night at Blackpool.”
TAKEN FROM THE BBC SPORT WEBSITE