Thapanee Kiatphaibool, governor of the Tourism Authority of Thailand, forecast the figure could rise to nearly 1.2 million next year, Bangkok Post reported.
About 800,000 American tourists have visited Thailand this year, a 5% increase year on year.
Thailand, the second largest economy in Southeast Asia, welcomed 1.03 million American tourists last year, generating 58.8 billion baht (US$1.8 billion).
The expansion of flight services between the U.S. and Thailand has been one of the key factors behind the growth, Aviation A2Z reported.
United Airlines recently resumed its service connecting Bangkok with Los Angeles via Hong Kong using 257-seat Boeing 787-9s.
The route marks the carrier's return to the Thai market after discontinuing its Narita–Bangkok connection in 2014.
Alaska Airlines has expressed interest in opening new routes between the U.S. and Thailand.
The U.S. is a high-spending market with strong demand from premium travelers, boosted by the popularity of season 3 of "The White Lotus" TV series, which was primarily filmed in Thailand, Kiatphaibool said.
According to the Tourism Ministry, American tourists had an average length of stay of 12 days, and daily expenditure of 4,726 baht.