The 22-year-old, who made his first-team debut under former Blues boss Carlo Ancelotti in September 2010 in the Champions League, racked up 22 appearances with the majority of them for the Italian.
He signed a new five-year contract with the Stamford Bridge club at the age of 18 in 2011 but has seen a once highly-promising career stall with a series of loan moves.
He spent last season at Vitesse Arnhem in Holland and has previously appeared for Swansea, Middlesbrough, Watford and Wigan.
McEachran was part of the England Under-17 side which won the European Championship in 2010 and won the FA Youth Cup with Chelsea in the same year.
Brentford are looking to build on a fifth place finish in last season’s Championship.
McEachran would be new head coach’s Marinus Dijkhuizen latest recruit after the signings of defenders Yoann Barbet, Andreas Bjelland and midfielders Akaki Gogia, Ryan Williams and Konstantin Kerschbaumer.