The 22-year-old has agreed a four-year contract at the City Ground, with Forest holding an option to extend the deal by another season.
Diomande becomes Forest's third addition of the transfer window following the arrivals of Portuguese midfielder Gustavo Sa and Austria international Xaver Schlager, who joined on a free transfer from RB Leipzig.
His move to England reunites him with Forest head coach Oliver Glasner, who has been a long-time admirer of the centre-back. Glasner previously attempted to sign Diomande while managing Crystal Palace.
Diomande joined Sporting from Danish side FC Midtjylland in January 2023 after beginning his career with Olympic Sport Abobo in Ivory Coast.
He went on to make 132 appearances for the Portuguese club, scoring seven goals and helping them win consecutive Primeira Liga titles in 2023-24 and 2024-25. He also lifted the Taca de Portugal and played a key role as Sporting reached the Champions League quarter-finals last season, where they were eliminated by Arsenal.
His international career has also brought significant success. Diomande was part of the Ivory Coast squad that won the Africa Cup of Nations in 2024 and has since collected 15 senior caps.
He featured once at this year's World Cup, appearing in the group-stage victory over Curacao.
Diomande is now looking forward to beginning the next chapter of his career at Forest.
"I am very happy to be here. It is a historic club, and it motivates me to join such a club," he said.
"I believe we can achieve very good things this season and I am looking forward to meeting the players and head coach.
"I will give my best and all I can to help the team. I cannot wait to meet the supporters at the City Ground and together we can have a good season."
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