Fletcher, 20, signed for the Hammers on a free transfer from Manchester United last summer, but has since struggled to make an impression at the London Stadium, making just two starts all season and scoring once – that being against his old club in November.
With West Ham looking to recruit a high profile striker this summer, Fletcher could be allowed to leave the club should the right offer come in.
Watford join a number of clubs interested in the former Barnsley loanee, with Burnley and Huddersfield also keen to take on the striker. The Terriers are thought to be preparing a £5 million bid for Fletcher as they look to bolster their attacking options ahead in their inaugural season in the Premier League.
Fletcher will re-join his team-mates at West Ham for pre-season on July 15th.