The 21-year-old has arrived from Belgian side Lokeren on a four-year deal, with the option of a further year.
His arrival follows the addition of Aapo Helme and Toby Sibbick, with Diaby being the club’s eighth signing of the summer so far.
Diaby will complete for a place in tbe back four alongside Halme, Sibbick and Danish recruit Mads Juel Andersen as the club build a new back four after the big-money exits of Ethan Pinnock and Liam Lindsay.
“I know it’s a new division for me and the team, but when I saw the team in the dressing room, the one thing that was very important is that the team was together,” said Diaby.
“Everyone is young but smiling and enjoying themselves and that’s important for a good season. The first impression I got is that (Daniel Stendel is an exciting coach.
“I like this because he will encourage you even if you make a mistake. When I was speaking to him, he said he needs my power and that makes me happy.”
Co-Chairman Paul Conway said Diaby was another strong addition to the side and further evidence of the club investing money back into the team following the sale of promotion-winning players earlier this summer.
“Bambo has deep experience at centre-back at a high level, at a young age,” said Conway.
“As promised, we are aggressively reinvesting the proceeds from the sale of Ethan Pinnock and have acquired three centre-backs in three days, since his departure.