The 22-year-old can play on either the right or the left wing and joins the Terriers on an initial deal until the end of the 2018-19 campaign, with the option to make that switch permanent next summer.
Mbenza, who is a Belgium Under-21 international, will wear the number 18 shirt at the John Smith's Stadium and could make his debut at home to Chelsea in the opening match of the season on Saturday.
"Isaac is a very exciting player and this is a great deal for Huddersfield Town for a player with such ability and potential. You do not play for Belgium at Under-21 level unless you have some outstanding attributes," manager David Wagner told the club's official website.
"He's a very quick winger with a direct style of play, which is exactly what we were looking for. As you can see from his last season at Montpellier, he has a real eye for goal too.
"When we first spoke, it's clear that he is a real winner too; a fierce competitor who will not accept second-best. I like these characteristics in our players. I'm looking forward to working with Isaac on the training pitch and in the classroom to help him understand our style of play."
Mbenza scored eight goals in 38 Ligue 1 appearances for Montpellier last season.