The Turkey international, who joined the south-coast side permanently from Getafe in July, has made 17 appearances this season, scoring two goals.
It comes a day after Bournemouth confirmed fellow striker Evanilson had undergone surgery to repair a fractured metatarsal sustained during Saturday's 1-0 win over Everton.
"We are devastated to confirm that Enes Unal has suffered a tear to his right anterior cruciate ligament during a training session this week," Bournemouth said in a statement.
"The Turkish striker, who has featured in every Cherries game since September, scoring two vital goals in our current club-record unbeaten run in the Premier League, experienced the contact injury in a routine drill on Tuesday morning."
The club said Unal would undergo surgery in the coming days.
Unal, 27, who suffered a broken toe in the lead-up to Euro 2024, also damaged an ACL in 2023, sidelining him for months.
Bournemouth are seventh in the Premier League table, just three points outside the top four.
