The 22-year-old was wanted by David Wagner as the Town chief looks to add more firepower to his squad ahead of the Premier League season.
Diakhaby can play either out wide or up front and is a former France Under-21 international. He joined Monaco last summer for £9m from Rennes but struggled, scoring just three times in 30 appearances.
Wagner, however, believes the Ajaccio-born attacker can follow former Monaco team-mate Terence Kongolo, Town’s record £16m signing, by making a big impact in English football. Diakhaby trained with his new team-mates for the first time last night.
Yorkshire’s sole top flight representative were the joint lowest scorers with Swansea City in 2017-18 after finding the net just 28 times.
Huddersfield, meanwhile, continue their pre-season preparations this weekend with three games.
The Terriers will face Real Betis this afternoon in the Interwetten Cup semi-finals in Germany and then later tonight one of either Werder Bremen or local side Rot-Weiss Essen in the third-place play-off or final of the one day tournament. All the games, played at the 20,000-capacity Stadium Essen, will be 45 minutes long.
Then, tomorrow, Huddersfield tackle Bundesliga 2 side SV Darmstadt 98 in a friendly. This contest, played in Darmstadt, will be 90 minutes long.
Town travelled to Germany last week with a squad of 29. Aaron Mooy, who was given an extended holiday following his exertions in the World Cup, has since flown out to join up with his team-mates.
Wagner plans to field what will effectively be two different teams. Mooy is unlikely to feature along with Danny Williams and Ryan Schofield, the pair still being on their way back from long-term injury.
Erik Durm, the Germany international full-back signed from Borussia Dortmund last week, will also not be risked as the Terriers look to manage his return to action after missing all of last season through injury.
But the supporters who have followed their team to Germany may get a first glimpse of £5.7m summer signing Ramadan Sobhi.
After representing Egypt at the World Cup, he was given additional time off but has been training for the past week.
Still missing are World Cup duo Jonas Lossl and Mathias ‘Zanka’ Jorgensen, who will return to training early next week.