Thailand's national police chief says a fire at a hotel in the southern city of Hat Yai on Saturday was caused by a car bomb.
At least three people died, including a Malaysian tourist.
More than 300 people were hurt, mostly as a result of smoke inhalation.
The bomb came shortly after twin blasts killed 10 people in Yala city.
The BBC reports these were the most serious attacks in southern Thailand for some years.
Police believe the same group of Muslim insurgents was responsible for them both.
The decision to target the southern city of Hat Yai is seen as a significant escalation.
It's one of the largest cities in Thailand and is a popular destination for tourists from across the nearby border with Malaysia.
The fire at the Lee Gardens Plaza Hotel there was initially blamed on a gas leak, but police later found it was a car bomb.
Thailand's southern provinces have been plagued by bomb attacks and shootings since 2004, when a Muslim separatist campaign reignited.