Police arrived at the crime scene, a townhome in Khan Na Yow district, to find 21-year-old actor Thanapat Chanakulpisan waiting to surrender. He was holding the weapon at the time.
The corpse of his girlfriend, Chatsaran Suwannakit, 25, was found on the floor with some 20 stab wounds on her chest.
The actor reportedly told police he and his girlfriend had been quarrelling and she picked up the knife from the kitchen counter.
He said he tried to take the weapon from her for fear she would injure herself but accidentally stabbed her to death in the process.
Police, however, have dismissed his explanation.
Initial investigation found that Chatsaran had posted an image of a medical certificate declaring she was clinically depressed on July 8. The image was captioned: “Getting hurt by lovers in the past and present has caused depression and made me bipolar for more than two years.”