The Black Cats accepted an offer from the Reds earlier this week and allowed Henderson to head to Merseyside for talks, The midfielder has already joined the England Under 21 squad in Denmark ahead of the start of the European Championship this weekend.
Henderson is in line to face Spain on Sunday and will remain focused on international duty until later in the summer.
"I'm over the moon, overjoyed to be here and I'm really looking forward to it," he told the club's official website.
"Liverpool's definitely the place to help me come on as a player, and improve me. The club's going in the right direction and it can go as high as it wants."
Henderson is looking forward to playing alongside stars such as ndy Carroll and Luis Suarez, as well as captain Steven Gerrard.
Speaking about Gerrard he said; "He 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."
"I've trained with him once (for England) and he was unbelievable, to be honest.
"I'm looking forward to it, and there are other good players here who can help me as well.
"Steven has been a massive, massive player for Liverpool for years now and he still is. Hopefully I can learn a lot from him.
"Coming to a massive club like Liverpool, there is always going to be competition.
"Hopefully I can keep working hard, keep improving, and get my chance on the pitch.
"There is a lot to achieve. It's a massive club and we've seen over the last few months how good they are.
"So I'm just coming here to learn and try to improve my game and I'm sure that along the way there will be a lot of happy memories.
"You come to a big club and you want to win things. Liverpool have won a lot in the past and hopefully there is a lot more to come in the future."
