Henderson has put pen to paper on what the Merseyside club have described as a "long-term deal", having agreed personal terms and visited the Reds' Melwood training complex before undergoing a medical.
The highly-rated 20-year-old has moved for an undisclosed fee, with reports suggesting it could be up to £20million.
Henderson told the club's official website: "I'm over the moon, overjoyed to be here and I'm really looking forward to it.
"I was flattered to know Liverpool were interested in me and, once I knew, it was a massive decision for me to make but also a massive opportunity.
"Obviously it's hard to leave your local club. I'm a Sunderland lad, I've supported them all my life.
"But I'm really looking forward to the future now and obviously this is a massive opportunity for me. I'm really excited by it."
In total he has made 79 appearances for the Wearsiders, scoring five goals, and he also spent a spell on loan at Coventry in the 2008-09 season.
He said: "My family are all Sunderland fans as well, and it's pretty tough because I've had some great years there.
"I've been there since I was seven and I'll definitely miss it but, like I say, I'm looking forward to playing for Liverpool now."