Chris Wilder expressed his delight at securing the services of Jack O'Connell until 2022.
Speaking upon confirmation of O'Connell's new four-and-a-half year contract being signed off, the Blades boss remarked: "Jack has impressed everybody, there is only one opinion regarding Jack.
"He is an important player who has done fantastically well having come from Brentford's.
"He has adapted to the step up this season and has been outstanding for us."
Wilder added: "I wanted to keep him, he wanted to stay and it's part of our medium to long-term strategy of keeping our best players.
"We've got a group to build around and Jack's firmly one of those. He has had to perform to remain in the team, but he is a proper footballer, he trains as he plays, and he is an excellent professional.
"He is great to work with and these of the type of players we've brought to the club to improve and develop with us."