His three-day tour to southeast Asia will also take him to Myanmar, the first by a US president, a hitherto closed country which is slowly opening up to the world.
Obama called on Thai King Bhumibol Adulyadej and held a bilateral meeting with Prime Minister Yingluck Shinawatra.
The President was greeted by 40 saluting military guards who flanked both sides of a red carpet.
Obama and US Secretary of State Hillary Clinton visited the Wat Pho Royal Monastery, which houses a gigantic reclining Buddha and a towering seated Buddha.
It is Obama's first trip abroad after winning his second term. On Tuesday Obama will visit Cambodia to attend the summit. Thailand is a major non-NATO ally.
"It was very important for us to send a signal to the region that allies are going to continue to be the foundation of our approach" to establishing a more prominent presence in Asia, Deputy National Security Adviser Ben Rhodes told reporters traveling with the president aboard Air Force One.
Obama is also seeking to open new markets for US businesses; the United States is Thailand's third biggest trading partner, behind China and Japan.
Becoming a counterweight to China in the region is a keystone of Washington's new policy for Asia-Pacific.
In Myanmar, Obama will meet Noble laureate Aung Saan Suu Kyi. The focus of the trip to the region is also to take advantage of the economic and trade opportunities in this region.