The 19 year-old has extended his deal until 2018, having become a regular part of the first-team squad over the past year.
Since Ronald Koeman’s arrival this summer, the England Under-19 international has played in all of Saints’ pre-season friendlies so far and looks set to play a big role under new the new head coach.
“I’m over the moon to sign for this club for the next four years,” said Reed.
"I think it is a positive signal to everybody that we have secured Harrison for another four years"
“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.”
Speaking to SaintsFC.co.uk, Reed added: “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.”
After securing the talented youngster’s signature, Koeman said: “This is very good news. I think it is a positive signal to everybody that we have secured Harrison for another four years.
“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.”