Wirral-born Charsley leaves Goodison on a short-term deal after making his first-team breakthrough in December. The Republic of Ireland under-21 international was part of the team that started Everton's Europa League game in Cyprus against Apollon Limassol last month.
Charsley, who played at left-back that night, was a key member of Everton's Premier League 2 title success but a loan move has been in the offing for some time as the club look to accelerate his development.
“We have brought him in until the end of the season,” confirmed Trotters boss Phil Parkinson. “We're a little short in the midfield department.
“Harry can play in the wide positions, he can play in behind the striker. He's a very versatile player and he's got a real energy about him. He will go straight into the squad for Brentford.”