Henderson, 36, captained Liverpool to the Premier League title in 2019-20 and also won the Champions League, FA Cup, League Cup twice, the UEFA Super Cup and the FIFA Club World Cup during his time at Anfield.
He left for Saudi Arabian club Al-Ettifaq in 2023, before joining Dutch side Ajax in January 2024, and returned to the Premier League with Brentford last summer, making 32 top-flight appearances in 2025-26. He becomes Chelsea's eighth first-team signing since Xabi Alonso took charge, and, at 36, the most senior player recruited by the club's BlueCo ownership since their 2022 takeover.
"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.
"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 move carries deeper resonance for Henderson than a standard transfer. Stamford Bridge was the scene of his professional debut, on a torrential November night in 2008, when Sunderland manager Roy Keane sent on an 18-year-old midfielder at half-time with his side already 3-0 down.
"It feels like a bit of a full-circle moment," Henderson says, recalling the occasion. "I can remember getting beat 3-0 at half-time. It wasn't going great, and Roy Keane said: 'Get yourself warm, you're going on for the second half.' I went out, warmed up, and got ready to come on.
"To play in the Premier League at Stamford Bridge against an amazing team with amazing players, Lampard, Ashley Cole, Terry, Drogba, was an incredible moment for me. We got beat by five, didn't we?! So not the greatest game, but what an amazing feeling. To then be joining the club quite a few years later is pretty special."
Chelsea won that match 5-0; 784 club and country appearances later, the stadium he first played in as a professional is now his home.
That same weekend in 2008 also saw Chelsea's 86-game unbeaten home league run, stretching back four and a half years, ended by Liverpool, with the only goal scored by Xabi Alonso. The pair overlapped in the Premier League for a single season before Alonso's playing career wound down in England, and they will now work together for the first time as manager and player.
"I had huge admiration for Xabi as a player, and now I have huge admiration for him as a manager," Henderson said. "What he's done over the last few years has been pretty special. Everyone that I know that has either worked with him or played with him has said very good things. Not one person had a bad thing to say about him. When I spoke to him, I got a very good feeling that it would be an honour to come and play under him."
Henderson will report for training at Cobham once his summer break ends, having reached the final weekend of the World Cup with England before suffering a broken wrist during the squad's celebrations following their last-16 win over Mexico at the Azteca Stadium. He says the injury is healing well and that the enforced rest has left him fresh.
Reflecting on a "brilliant" season at Brentford that helped earn his World Cup squad place, he added: "The main goal is to be competitive, and to be successful. There is a lot to look forward to, but a lot of work to be done as well. It's about giving everything and being hard to beat every time we play. I've dedicated my life to the game ever since I was a kid, and that's never changed, really. I just try and focus on being the best version of myself and helping the team as much as possible."