A young man has been arrested after his 21-year-old dating app lover was found strangled to death shortly after learning he already had a wife.

Police arrested Anuwat Pimmat, 22, in Prachuap Khiri Khan about 10pm on Sunday.

His arrest followed the discovery a young woman's body among the weeds alongside the Phetkasem Highway in Khao Yoi district in Phetchaburi on Friday. The victim was identified as Benjarat Choeijan, aged 22, from Phuket province.

The suspect told police he had met Benjarat through a dating app and had developed a serious relationship with her since the Songkran festival in April.

He took a flight from Bangkok to Phuket to meet her and she had picked him up from his hotel in a car. They had then gone to Prachuap Khiri Khan to collect a pickup truck they planned to return to her former boyfriend in Bangkok.

As they were passing through Sam Roi Yot district of Prachuap Khiri Khan on June 3, Mr Anuwat answered a phone call from his wife. When Benjarat found out he was not single, she had shouted at him and slapped his face, Mr Anuwat said. He became furious and strangled her.

According to police, Mr Anuwat said he then drove around looking for a place to dump the body. It was discovered four days later, on Friday.