They included 1.75 million Chinese tourists, the highest from any country, according to data from the Thai Ministry of Tourism and Sports.
Malaysians were second at 1.16 million, followed by Russians at 622,800.
The rising number is due to the government's visa waivers for the nationals of many countries, the ministry said.
Besides, it recently unveiled a scheme that covers medical expenses of up to 500,000 baht ($14,000) in the event of an accident and pay compensation of up to one million baht in case of death.
Some 28 million people visited Thailand in 2023, up from 11 million the year before, but still well down from the 40 million who came in 2019, the year before Covid.
Officials have set their sights on 35 million arrivals in 2024 and tourism revenues of $55 billion.
The number of foreign tourists to Vietnam in the first quarter was 4.6 million, 3.2% more than in 2019.