Dennis burst onto the international scene last season but failed to make an appearance during the recent Test series with the British & Irish Lions.
However he has penned a new two-year deal, which takes him through to the 2015 World Cup in England.
"I really want to keep working hard to try and get back in the Test fold and contribute to winning a first ever Super Rugby title with the Waratahs," said Dennis.
Meanwhile Horne, who came off the bench in the second Test against the Lions, has signed on for one year.
The 15-cap centre was plagued by injuries last season and is hoping to have a clear run of matches in the new campaign
"I'm really pleased to be staying in Australia, especially with so much to look forward to in the coming season," sais Horne.
"I am really excited at what I think the NSW group can achieve."
The star duo join fellow Waratahs Sekope Kepu, Wycliff Palu, Tatafu Polota-Nau and Benn Robinson in re-signing with the ARU.