The 20-year-old Frenchman, who can play at centre-half and left-back, came through the youth ranks at Nottingham Forest.
He graduated into the reserves, where he played alongside current Silkmen Emile Sinclair and Hamza Bencherif.
Diagne left Forest at the end of last season and has since been playing back in his homeland with Parisian side FC Municipal d'Aubervilliers.
Macclesfield manager Gary Simpson said: "I'm delighted that Tony has committed himself to the club. He's been playing back at home in Paris but I know he was keen to have another go after things didn't work out for him at Forest.
"He paid for his own flight to come over and whilst I would usually expect to have a look at players in competitive games, he did enough in the two 11 vs 11 games we had amongst ourselves last week, to tell me he'll be a great addition to the squad."
Source: PA
Source: PA