A phone app reveals the murder tale of a South Korean professor trying to get out of his marriage and keep his mistress.

The professor sent a message to his mistress through the app used by millions every day, saying "brace yourself".

The message has become key evidence in a murder case, with the professor arrested for killing his wife and his mistress questioned as an accomplice to the plot, media reported on Tuesday citing local police source.

The professor went to the head office of the "Kakao Talk" messenger service provider after the murder and asked them to delete the message. But the message was saved for a month and police were able to retrieve it as part of an investigation of his phone records.

The professor has admitted and confessed to the murderer of his wife. He said he strangled her and used the mistress's car to take the body to the sea and dump it into sea. He and his wife were in the process of getting a divorce.

"The message helped us discover that the woman was his accomplice," a spokesman at a police station in the southern city of Busan said.

"Kakao Talk" is a relatively new free smartphone app with over 14 million users that allows them to send messages, chat, take part in group chats, and send photos.