The free tourist visa programme waives the 1,000-baht (28.50-dollar) fee for applications abroad and halves the visa on arrival to 1,000 baht. The programme was initially planned to end in May.
The 21 countries include Andorra, Bhutan, Bulgaria, China, Cyprus, Ethiopia, Fiji, India, Kazakhstan, Latvia, Lithuania, the Maldives, Malta, Papua New Guinea, Romania, San Marino, Saudi Arabia, Taiwan, Ukraine and Uzbekistan.
Last week, the Tourism Authority of Thailand (TAT) forecast a 10 percent growth in tourist spending this year at more than 2.77 trillion baht, up from 2016's 2.52 trillion baht. According to TAT, Thailand hosted nearly 33 million foreign visitors last year.
The stimulus measure came after waning tourist arrivals, especially from China, as the Thai government cracked down on cheap tours, pressuring Chinese tourists in Thailand to buy products or services not included in their programmes.