The 19-year-old can play across the midfield and has been with The Hornets for five years, turning professional in 2017.
Charles has had experience with the Watford first team through friendlies in the summer of 2018.
Speaking on his arrival, Charles is delighted to be a Bee!
“I’m very happy to be here and to get the opportunity to hopefully get some first team football at a great club.
“The club has been on a good cup run this season which caught my eye, hopefully, that keeps going and I can be a part of that.
“The manager Darren has got a lot of playing experience under his belt and I’m looking forward to learning from him.
“I’m very thankful to him and the club for giving me this opportunity.”
Welcome to Barnet, Ashley!