The 22-year-old striker made his Ligue 1 debut in 2016 for Rennes.
He scored his first goal just two weeks after his debut away to Montpellier.
He then joined Monaco in 2017, and has made 16 appearances and scored two goals.
Diakhaby has also made four appearances for the U21 French national team.
Huddersfield kick off against Chelsea in the Premier League on August 11.