The France international has fielded enquiries this summer from across Europe.
But Sport says Barca have informed his agents that he is not for sale.
The word came from management hours after Dembele's stunning goal in the Supercopa triumph against Sevilla in Tangier.
Dembele's appearance was proof of the confidence coach Ernesto Valverde has in the attacker.
A source stated: "He played as a starter, although he was one of the last to return from vacation."