The blast struck a pharmacy inside the King Mongkut Hospital, stirring panic among patients and sending smoke into the corridors but causing only minor injuries.
Police in Thailand say 24 people have been injured in a bomb blast.
Investigators found the remains of wires, a circuit board and batteries at the scene, police said.
Government spokesperson Sansern Kaewkamnerd said 24 people had been wounded.
Kamthorn Aucharoen, the commander of police's explosive ordnance team, told media on Monday that an explosion had been caused by a bomb at the Phramongkutklao Hospital.
Authorities blamed that attack on Muslim insurgents.
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New York-based Human Rights Watch condemned the hospital attack as "a cruel and inhumane action which grossly violates human rights", Sunai Phasuk, the group's senior Thailand researcher said on Twitter.
Police are already hunting suspects behind two other small blasts in recent weeks, but have given conflicting and contradictory information over the devices and likely suspects.
The explosion at the Phramongkutklao Hospital reportedly occured on the first floor room of the Chalermphrakiat building, in a reception room for retired civil servants at about 11am Monday, the same day Thailand marked the third anniversary of a coup d'etat that brought the current military regime to power.
Since the coup, the junta, known as the National Council for Peace and Order, has clamped down on dissent and ramped up prosecutions under sedition and royal defamation laws. Last month, a similar explosion took place in front of an old government lottery office, also wounding two.
Earlier this month, a vehicle bomb at a shopping centre near Thailand's border with Malaysia wounded 61 people. Last Monday, a bomb went off in front of the country's National Theater.