Aaron McLean's goal was enough to give Hull their second successive victory and condemn Peterborough to a second straight defeat.
McLean, who swapped Posh for the Tigers last year, netted from Tom Cairney's corner just after half-time.
Earlier, Grant McCann could have given United the lead in second minute, but rifled his shot over the bar.
Emile Sinclair and Ryan Bennett had Peterborough's best chances but neither could make them count.
After McCann fired over, Martyn Waghorn had an opportunity to give Hull the lead, but his effort was palmed onto the bar by goalkeeper Paul Jones.
Later, Hull could have made the scoreline look much more flattering had they not found Jones in good form, with the Posh keeper saving well from Jack Hobbs' header.
At the other end, Adriano Basso was not to be outdone as he brilliantly denied Craig Alcock and Nigel Pearson's side held on for all three points.
Posh boss Darren Ferguson told BBC Radio Cambridgeshire: "It was a game we certainly didn't deserve to lose. It should have been 0-0 at the very least.
"As the ball dropped to Aaron I just had a feeling he'd score as I've seen him do it before.
"If I'm going to be critical of my side, then perhaps we didn't get enough shots off."