The 21-year-old is a versatile attacker who can play as a second striker or a winger.
Both Benfica and the Baggies have an option to review the loan deal in January.
Brother of Joe Willock, who still plays for Arsenal, Chris came through the Gunners academy and made his senior debut as a 16-year-old in the Carabao Cup.
He signed a five-year deal with Benfica in 2017 but has not yet played for their first team.
Willock has been playing for Benfica B, who he scored 11 goals for last season.
Technical director Luke Dowling said: “We’ve got a very good deal to bring him here for the season and Chris is excited at the opportunity.
“He’s been looking to get back over here and Slaven enjoys working with and developing youngsters.
"It will be a brilliant shop window for him. He’s an exciting forward player with the capacity to work in the wide or central areas.”
Albion could yet still complete a loan deal for Sporting winger Matheus Pereira today.