Cantonal police said on Thursday that an investigation has been launched into the fight, involving three people at the bordello in the centre of the Bernese Oberland town.
Officers were notified of a punch-up in a hallway of the property on the Obere Hauptgasse in Thun on Wednesday at around 9pm.
The alleged instigator has been arrested after being treated for minor injuries and has admitted to his involvement in the altercation, police said.
Police confiscated a knife and were looking into whether it was used against the seriously injured victim.
One other man received minor injuries in the fight.
No women were reported hurt in the incident.
Investigators are still looking into the reason for the confrontation.
They are also seeking to clarify whether the man being detained was the owner of the knife, cantonal police spokesman Andreas Hoffman told the SDA news service.