Toby Stevenson has signed a one-year deal with the Hornets, with the option of a further year.
The 20-year-old left-back came through the academy at Leyton Orient before spending time with Charlton Athletic.
On his Charlton debut in October 2018, Stevenson netted a hat-trick in an 8-0 Checkatrade Trophy win against Stevenage.
He joins Watford's academy after his Charlton contract expired earlier this summer.
