The 26-year-old, who is of Congolese descent, completed his expected move to United today and will boost Leeds' options in the engine room.
His arrival follows on from United’s capture of his Palermo team-mate and close friend Sol Bamba, with the former Leicester City central defender, 30, also signing on for Leeds for the reminder of the campaign.
On sealing his move to Leeds, Granddi said: “I’m happy to be here. I’m very happy to be joining a good group. I will do my best for this club. I’m happy because I didn’t play a lot of games in Palermo, I’m very happy.”
Granddi, who has been capped five times for the French under-21 side, started his career at Paris St Germain.
After loan spells with Clermont Foot, Brest and Nantes, Ngoyi joined Troyes on a permanent basis in June 2013, signing a three-year deal with the newly-promoted Ligue 1 club after leaving Paris on a free transfer.
The 6ft 1in midfielder left Troyes following their relegation from the French top-flight in the summer of 2013, penning a four-year deal with the Sicilian outfit, who went up to Serie A in 2012-13.
Ngoyi has found himself out of favour at Palermo this season and has played just 71 minutes of first-team football.