It is the second time that manager Chris Wilder has loaned the player from Millwall after Marquis played his part in the run to Football League safety in 2013/14.
An Achilles injury has ruled leading goalscorer Richards out for around four weeks and Marquis, out of contract at Millwall this summer, will look to impress potential suitors and help Northampton seal promotion over the next few weeks.
Wilder, who takes his side to York City on Tuesday night, told the official club website: "John is a player the supporters know well and he was very popular here in his first spell. He is a fighter, a striker who has a physical presence and can finish."
The trip to York, originally scheduled for Boxing Day, means a quick rematch with the Minstermen after Northampton's comfortable win at Sixfields at the beginning of February.