The company did not give a figure for the planned investment in Thailand's Rayong province, southeast of Bangkok.
Katie Whitmore, Harley-Davidson public relations manager, said the company had its best results in Asia-Pacific in 2016, though she gave no numbers.
The Thailand facility "will allow us to be more responsive and competitive in the Asean region and China," Harley-Davidson public relations manager Katie Whitmore said.
"Increased access and affordability for our customers in the region is key to growth for the company in total," she said. "There is no intent to reduce H-D U.S. manufacturing due to this expansion."
The plant would let Milwaukee-based Harley-Davidson avoid Thailand's up to 60 percent tariff on imported motorcycles and help it get tax breaks when exporting to Thailand's neighbors, thanks to a trade arrangement among members of the Association of Southeast Asian Nation (ASEAN).
Harley opened a plant in India in 2011. It also assembles motorcycles at a plant in Brazil.
After the New York Times reported on Harley's planned Thai investment, United Steelworkers (USW) International President Leo W. Gerard on Tuesday said the decision was "a slap in the face to the American worker and to hundreds of thousands of Harley riders across the country."
USW represents members at Harley plants in two U.S. states and 850,000 workers in North America.
Gerard also said that production outside the U.S. "puts in jeopardy the success that has propelled Harley over the years."
Whitmore said motorcycles assembled in Thailand would have the same "authentic look, sound and feel" as those manufactured in the U.S.
Demand for Harley motorcycles in the U.S., the company's biggest market, continues to be slow as its loyal baby boomer demographic changes ages.