China has been the biggest source of visitors with 2.9 million, followed by Malaysia, Russia and India, according to the Tourism Authority of Thailand.
The Thai government has attributed the excellent performance to its sweeping changes to visa policies.
In March Thailand permanently waived visas for Chinese nationals and extended the exemption for Russians, Indians and Taiwanese.
From June Thailand will allow travelers from 93 countries to enter without a visa for up to 60 days, up from the current 57 nations, as it strives to hit a target of 36 million visitors this year.
Vietnam only received 7.6 million foreign visitors in January-May.