Police have arrested three people in connection with the attack in Hua Hin, 200 kilometres south-west of Bangkok, local Police Chief Chayakorn Siradecho told dpa Thursday.
"We are now looking for the last suspect and we have a warrant out for his arrest" on assault charges, he said.
Three tourists were left lying motionless on the street after being punched and kicked in a sudden attack by a group of men on April 13.
CCTV footage of the incident has gone viral online.
Thailand's reputation as a tourist haven has suffered a setback over the past several years, due to incidents including the high-profile murders of two British backpackers on the resort of Koh Tao in 2014 and a bombing at a popular Bangkok tourist site in August that killed 20 people.
- DPA