The US president paid a courtesy call on the 84-year-old King, the world's longest-reigning monarch, at Siriraj Hospital where he has been staying for nourishment to regain his strength since September 2009 when he was admitted with fever, fatigue and a lung inflammation.
Obama later held talks with Thai Prime Minister Yingluck Shinawatra before a joint press conference and an official dinner at Government House.
Earlier Sunday, the US president visited Wat Pho Royal Monastery upon arrival. A monk gave him and Secretary of State Hillary Clinton a private tour of the temple.
After Thailand, the US president will visit Myanmar and Cambodia to attend the 4th ASEAN-US Leaders' Meeting and the 7th East Asia summit.