The move follows on from Town's capture of former goalkeeper Jonas Lossl, who has re-joined the club on loan for the rest of the season from Everton - having joined the Merseysiders on a three-year deal in the summer.
Willock, who can operate down both flanks, has joined the Terriers on his 22nd birthday.
The Londoner spent the first half of the campaign on loan at West Bromwich Albion and featured heavily for their PL2 side. In 10 games he played for the Baggies development squad, he was involved in nine goals; scoring four and setting up five.
The winger began his career at Arsenal and made two senior appearances before joining Benfica in the summer of 2017. He has gone on to make 64 appearances for Benfica B scoring 14 goals.
Willock has also been a regular for England’s youth sides, being capped at every age group between Under-16 and Under-20.
Town chief Cowley commented: “Chris is a young player with huge potential. He’s a skilful wide player that can play off either side.
“He’s good in the one vs one moment and is capable of getting in behind defences with his movement. He’s a creative player and will give us something different in the attacking third of the pitch.
“He comes with a point to prove and a real enthusiasm to play.”