The 23-year-old has had two previous spells at Griffin Park, making 12 appearances and scoring three goals, but was recalled from his last stint by the Lions in November.
"We were disappointed when Lewis had to return to Millwall but I'm delighted to be able to sign him now," said Bees boss Andy Scott.
"We know what he can give us and it's good to take on players who understand us, know what we're about and can fit in easily and help us through to the end of the season."
Source: PA
Source: PA