No fee or details of his contract were disclosed but Liverpool said Henderson had passed a medical and agreed personal terms to become manager Kenny Dalglish's first major signing since the end of last season.
Dalglish has identified the 20-year-old Henderson as a key player as he bids to bring the English league title back to Anfield for the first time since 1990.
"I'm over the moon to be here and the fans will see me giving everything I've got in every game I play," Henderson said on the Premier League club's website.
"Hopefully, they'll enjoy watching me as much as I'll enjoy playing for this club."
Newspaper reports said Henderson, capped once by England in their 2-1 friendly defeat by France last November, cost Liverpool around 20 million pounds and the player has now joined up with the England under-21 squad ahead of the European Championship in Denmark.
CONSISTENT SEASON
Henderson enjoyed a consistent season for Sunderland with a number of strong displays in the centre of midfield and is considered one of the country's best prospects.
He will now compete with Steven Gerrard, Lucas Leiva, Raul Meireles, Jay Spearing, Christian Poulsen and Jonjo Shelvey for a place in Liverpool's midfield.
"He (Gerrard) is one of the best players in the world, you want to be playing with him and training with him to try and improve yourself as a player," Henderson said.
"I've trained with him once (for England) and he was unbelievable, to be honest. Steven has been a massive, massive player for Liverpool for years now and he still is. Hopefully I can learn a lot from him."
Liverpool Director of Football Damien Comolli praised Henderson's maturity and work ethic.
"It is very difficult to believe you are talking to a 20-year-old. You think you're talking to a 25-year-old," Comolli said.
"He's very passionate about football, very committed, a top professional. He's the type of player who'll stay behind after training and do extra work."
Dalglish returned to Liverpool in January and although Spanish striker Fernando Torres left the club soon afterwards he was replaced by England forward Andy Carroll and Uruguayan Luis Suarez for a combined fee of around 50 million pounds.
