McEachran, who had loan spells with Swansea City and Middlesbrough earlier in his career, moves to Vicarage Road until January as Gianfranco Zola looks to lead Watford to promotion to the Premier League.
The midfielder made his debut for Chelsea in 2010 when he came on as a substitute in a UEFA Champions League win at Zilina, but is yet to establish himself in the first team at Stamford Bridge, making just 22 appearances - with 15 of those coming as a substitute.
He will now gain more first-team experience after joining a Watford side that has performed well in the early stages of the campaign, losing just one of their first seven encounters.
Zola's men sit sixth in the Championship as a result of their strong start, and they will hope that McEachran can provide his side with an added spark in attacking areas.
McEachran will join up with the Watford squad following their game with Barnsley at Oakwell on Saturday.
