Malaysia came in second with 2.43 million arrivals, followed by India with just over 1 million, according to data from Thai Ministry of Tourism and Sports.
Thailand received 17.5 million foreign arrivals between January and June, half of this year's target at 36.7 million.
Last year, Malaysia overtook China to become the largest feeder tourism market of Thailand.
Thailand made the waiver permanent for Chinese tourists in March.
Since June, Thailand has allowed travelers from 93 countries to enter without a visa for up to 60 days, up from the current 57 nations.
Vietnam received 8.8 million foreign arrivals in the first half, only half of Thailand's number.
In 2019, Thailand, whose economy heavily relies on tourism for economic growth, welcomed a record 39.9 million foreign arrivals, generating 1.91 trillion baht (US$51.9 billion) in revenue.