The annual Cobra Gold military exercise will be held from Feb 20 to March 5, but in a reduced format and without observers from Myanmar.
Details were announced at Royal Thai Armed Forces Headquarters by Chief of the Joint Staff Gen Natapol Boon-ngarm, accompanied by US charge d'affaires Michael Heath.
He said the number of troops would be reduced from 8,964 to 3,460 and participants would be subject to strict health measures to ensure control of Covid-19.
There would be no amphibious landing, civilian evacuation or live-fire exercises, and the number of troops participating in each drill would be capped at 500.
Gen Natapol and Mr Heath jointly denied a rumour the United States would use the exercise as an opportunity to secretly supply weapons to ethnic minority groups fighting the junta government in Myanmar.
Gen Natapol said he was aware of the rumour but Cobra Gold would not be held near the Myanmar border. The exercises would be in Prachuap Khiri Khan, Lop Buri, Rayong, Chon Buri and the upper Gulf of Thailand, he said.
Thailand and the US cooperated only on humanitarian aid and vaccine support for Myanmar, Mr Heath said.
Lt Gen Chitchanok Nujjaya, Director of Joint Operations, said Myanmar military observers would not participate in this year's 41st Cobra Gold, as they had in previous years, because of the Covid-19 situation. There was not enough time to prepare for Myanmar's participation, he said. This was a joint decision by Thailand and the US.
Twenty countries would take part in the exercise, the biggest annual military drill in Southeast Asia. Fully participating nations would be Singapore, Indonesia, Malaysia, Japan, South Korea, Thailand and the United States.
India, China and Australia would have limited participation, in human aid exercises.
Contingents from Bangladesh, Canada, France, England, Mongolia, Nepal, New Zealand, the Philippines, Fiji and Vietnam would participate in the Multinational Planning Augmentation Team project.
There would be 1,953 soldiers participating in this year's Cobra Gold from Thailand, 1,296 from the US, 50 from Singapore, 16 from Indonesia, 35 from Japan, 41 from South Korea, 36 from Malaysia, 10 from China, five from India and 18 from Australia.