Midfielder Calvert-Lewin has come through the youth ranks at Bramall Lane with flying colours and towards the end of last season, at Leyton Orient he made his debut.
Cobblers boss Chris Wilder said; "With Ivan (Toney) gone we were obviously looking to bring in another striker and we're delighted to have signed Dominic. He's very highly rated at Sheffield United, he's a great kid and hopefully he can add to what we've already got at the top of the pitch. We always look to unearth players that maybe a lot of people haven't heard about and that's what we've done with Dominic.
"We like what we've seen of him so far, he's enthusiastic and he's got the ability. We're not a fan of short term loans but we've got Dominic until January and Sheffield United know he will get well looked after. He's still young but we feel we can help him develop and he will definitely be involved and it's important that we have plenty of competition all over the pitch."
As part of the deal Calvert-Lewin will be granted permission to play in the Capital One Cup in their first round tie with Blackpool whilst the Blades also have a 24 hour recall clause once he has been at Sixfields for 28 days. He could make his debut today at league newcomers Bristol Rovers.