The 19 year-old has extended his deal with Saints until 2018, having become a regular part of the first-team set-up over the past year.
Reed has played in all three of Southampton’s pre-season friendlies, and is relishing the prospect of working under new manager Ronald Koeman.
“I’m over the moon to sign for this club for the next four years,” said Reed.
“I want to be at this football club because it’s only going forward and is going to keep pushing on, so I’m already looking forward to the future.
“I’ve really enjoyed the three weeks that I’ve been with the new manager and his coaching staff, so I’m looking to continue working on my game now.
“The quality of the training that the manager has put on and the ambition that he’s showing played a big part in my decision.
“The manager wants us to perform just as well and even better than last year, so I can’t wait to play a part in that.”
Koeman believes the club have signalled their intentions for the future by securing the England youth international on a new contract.
“This is very good news. I think it is a positive signal to everybody that we have secured Harrison for another four years,” Koeman said.
“We would like to keep all our good young players at Southampton, so I am very happy to be able to keep him here.
“Harrison has had some chances in the friendly matches this summer and has impressed me in those, along with the training sessions where he has shown his quality.
“He has good qualities and is a big talent. We know that he will develop himself even more as we progress, and will become a great player.”