The 22-year-old has put pen to paper on a four-year contract at the City Ground, with Forest holding the option of an additional twelve months, giving Glasner the commanding centre-back he has pursued for two years.
He arrives with genuine European pedigree, having made 132 appearances across all competitions for Sporting, including 87 in the Primeira Liga, and having gone deep into the Champions League with the Portuguese club. Diomande has also earned 16 caps for Ivory Coast, featuring at the 2023 Africa Cup of Nations, which his country won, and appearing at this year's World Cup.
"I am very happy to be here. It is a historic club, and it motivates me to join such a club," Diomande 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."
Forest's chief football officer, George Syrianos, said the club had been clear about the type of player it wanted to bring in this summer.
"We have been very clear on the profile of player we want to add to the squad, so we are excited to welcome Ousmane to the club," Syrianos said. "He has fantastic experience for a defender of his age. Playing a crucial role in title-winning teams and going into the latter stages of the Champions League puts him in a great position to integrate well here at Forest.
"Ousmane is a powerful, commanding defender, and arrives here with a winning mentality. He shares the same ambition for success we have, and we are excited to see the impact he can have in a Forest shirt."
Diomande also had a message for supporters after his move was confirmed.
"I want to say thank you, I've seen so many of the messages already and they were all so nice. I'm really looking forward to meeting them at the City Ground and hopefully we can have a good season together."
The move comes after reports last week suggested talks between Forest and Sporting had collapsed, only for negotiations to be revived within days. Diomande becomes one of Forest's marquee additions of the summer, arriving with a market value in excess of £36 million, as Glasner continues to strengthen his squad for the new Premier League campaign.