Barcelona on their club website revealed that Braithwaite's injury had not responded to "conservative treatment" so is expected to go under the knife in the "coming days."
"The first-team player Martin Braithwaite has a femoropatellar injury in his left knee which has not responded to conservative treatment.
"The Danish striker will undergo surgery in the coming days.
"The club will publish a new injury update when the process has taken place.
"The Danish international has made three appearances in the league so far this season totalling 226 minutes.
"On the opening day of the season against Real Sociedad at Camp Nou the striker scored twice and provided an assist for another goal. "
Braithwaite joins new signing Sergio Aguero who's nursing a calf injury as well as Ousmane Dembele and Ansu Fati on the sidelines.
The Denmark international started all three La Liga games this season and scored twice in Barcelona's opening-day win against Real Sociedad.
Braithwaite will miss Barcelona's Champions League opener against Bayern Munich at the Camp Nou on Tuesday.
Despite Barcelona being reluctant to disclose his return, it is believed that the 30-year-old will be out of action for between three and four months.
Braithwaite had a successful tournament with Denmark during the Euro 2020 but scored just a goal in six games.
Barcelona recruited Braithwaite from LaLiga side Leganes for a reported €18 million as he penned a four-and-a-half-year contract.
