The operator’s website – among the most-visited betting sites in the world – has been replaced with a single message, with access to all other pages blocked.
“Just like so many market-leading global organisations, Bet9ja recently became a victim of a sophisticated criminal cyber-attack, which is restricting your access to our platform,” it said.
The operator did not elaborate on potential sources of the attack, but added that player funds were not in danger.
“We take this matter extremely seriously,” it said. “Our priority is protecting our customers and you have our assurances that your accounts and all your funds are safe.
“We appreciate your patience and understanding at this difficult time.
“We will be back stronger as soon as is technically possible, rewarding you with a truly sensational bonanza of promotions.”