The 36-year-old is expected to complete his switch to Stamford Bridge in the coming days on a deal reported to run until June 2028, with Chelsea also working to add fellow experienced campaigner Danny Welbeck from Brighton as they look to blend youth with more senior figures in their squad.
Henderson made 34 appearances for Brentford last season after joining on a free transfer from Ajax, scoring once and playing a key role in the Bees' ninth-placed Premier League finish, before making history at this summer's World Cup by becoming the first England player to feature at seven major tournaments as Thomas Tuchel's side finished third.
Brentford director of football Phil Giles said the pair had always planned to discuss Henderson's next steps once the tournament was over, with manager Keith Andrews adding that the midfielder's experience had been invaluable during a season of transition following Christian Nørgaard's departure to Arsenal.
Henderson departs with Brentford's gratitude, having recovered from an arm injury sustained during the World Cup to be available for the new campaign, though it is Chelsea who will now benefit from his experience as the west London club continues reshaping its squad following the earlier arrivals of Marco Palestra and Morgan Rogers this summer.
Henderson joined Brentford last July after leaving Ajax, where he had served as captain, bringing more than 430 Premier League appearances of experience with him from his time at Liverpool and Sunderland.
"I leave Brentford with a lot of gratitude for the opportunity the club gave me, and for the support and care shown to me and my family from day one," Henderson said.