The move is reported to be worth around £3m, with a guaranteed fee in the region of £2.7m plus a further £300,000 in potential add-ons, and it takes the 18-year-old out of Gençlerbirliği's first-team squad in name only.
He will spend the rest of the 2026/27 season on loan back at the Süper Lig club, continuing to play regular senior football rather than dropping into Newcastle United's Under-21s.
The midfielder only moved to Turkey in December 2025 after coming through Académie SOAR in Guinea, but he has since made rapid progress, starting all three of Gençlerbirliği's league matches so far this season, including wins over Fenerbahçe and Göztepe, and playing the vast majority of the available minutes in central midfield.
His form has attracted interest from clubs well beyond Tyneside, and he added a senior international cap to his resume in June with his Guinea debut against Northern Ireland at Windsor Park.
"From the moment Newcastle showed interest, I knew it was the kind of ambitious club I wanted to be part of," Diabaté said.
"The project here is exciting, and the supporters have an incredible reputation. Feeling that energy behind me is something I'm really looking forward to."
Newcastle sporting director Ross Wilson confirmed the club had fought off competition from elsewhere in Europe to land the teenager.
"He's been on the radar of a number of clubs, so we're delighted he sees his future here with us — credit to our scouting team for identifying him early and working hard to get the deal done," Wilson said. "It was a pleasure to meet him recently and we are following his progress closely.
"His exposure to first-team football at such a young age, along with his recent international debut, shows the trajectory he's on. Going back to Gençlerbirliği will give him the chance to continue the development he's already shown while remaining firmly within our long-term plans."
Diabaté is understood to have signed a contract running until 2031, and the structure of the deal allows Newcastle to recall him from his loan as early as January should they choose to.
Gençlerbirliği currently sit joint top of the Süper Lig table after their opening three fixtures, and manager Metin Diyadin has spoken warmly of the player's rapid rise through the club's youth ranks before his breakthrough into the first team.