The 36-year-old has penned a two-year contract to take him through to the summer of 2028 and represents a huge shift in transfer strategy for the Blues.
Despite often focusing on youth recruitment, Chelsea have responded to Xabi Alonso's request for more experience by bringing in Henderson and Brighton striker Danny Welbeck.
"Given the size of the club, the manager, who I have great admiration for, and the quality of the players, this was a huge opportunity I couldn’t turn down," Henderson said.
"I was also so impressed with how much the ownership want Chelsea to be successful and move in the right direction.
"For me, it's about giving everything every single day, both on and off the field, to help the players around me and the team as much as possible. I'm very excited to get going."
The England midfielder has played 463 games in the Premier League. The majority of those appearances came for Liverpool, who he joined after breaking into Sunderland's first team as a youngster.
Spells in Saudi Arabia and with Ajax followed before Henderson returned to the Premier League with Brentford, where he played 34 games in all competitions last season and earned a call-up to England's World Cup squad.
It's unclear when his first game for Chelsea will be after the midfielder broke his arm while on international duty.