The 21-year-old, who spent a spell with Portsmouth earlier this season, is poised to go straight into their squad for the Devon derby against Plymouth Argyle at Plainmoor on Tuesday night.
Marquis' arrival comes after fellow forward Paul McCallum's loan deal from West Ham was ended by injury after three goals in four starts.
"He (Marquis) is not as tall as Paul McCallum, but he is more mobile," manager Alan Knill told the Herald Express. "He is also aggressive, with a good work ethic.
"He has played in winning Championship teams at Millwall, and he comes here with something to prove.
"Paul has done well for us, but he has had a back problem, which he's been playing with for a couple of games, and he wants to go back to West Ham to get it sorted out."