The Scotland international has signed a long-term deal at Anfield having completed a medical on Thursday, with his fee set to rise to as much as £10m should he meet certain criteria during his time on Merseyside.
Robertson's arrival could allow James Milner to move back to a more familiar role in midfield after he filled in admirably at left-back for much of last season, while Alberto Moreno and Jon Flanagan remain in Jurgen Klopp's squad.
The 23-year-old has played 57 Premier League matches for Hull, scoring one goal and laying on four assists, and was one of the Tigers's most consistent performers as they were relegated in 2016-17.
“It feels a wee bit surreal just now. Obviously I’m delighted," he told Liverpool's website upon his signing. "There’s been a lot of speculation over the last couple of weeks but I’m glad that the deal’s finally been done and I’m a Liverpool player.
“My family are proud of what I’ve achieved so far and they’re all over the moon with the move, as well as me of course. We’re all just looking forward to being part of the Liverpool family now.
“There’s not many, if any, more special clubs than Liverpool. When you grow up as a kid you dream of playing with big clubs such as Liverpool and to make that a reality is a dream come true for me.
“I just want to prove to people that I can do it at this level, and hopefully I manage to do that this season and go on to do good things for this club.”
The Glasgow-born defender started his professional career at Queen's Park after being released form Celtic's youth system. After one season in Scottish football's bottom tier, he joined Dundee United in the Premiership and moved to Hull City a year later.
He becomes Jurgen Klopp's third signing of the summer following the arrival of Dominic Solanke and Mohamed Salah earlier in pre-season.
After confirming Robertson's signing, Liverpool announced that midfielder Kevin Stewart has gone the other way for an undisclosed fee.
The 23-year-old signed a three-year deal with Hull after spending three years at Anfield, making 11 Premier League appearances, 20 in all competitions, over the last two seasons.