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.
The official added that police were investigating the incident at the hospital, which is frequented by retired military personnel.
"Right now, authorities are checking out closed circuit cameras,” he said.
Government spokesman Sansern Kaewkamnerd confirmed the incident, saying 24 people had been wounded.
There was no claim of responsibility for the blast.
May 22 marks the third anniversary of a 2014 military coup, which returned power into the hands of the junta following a cycle of mass protests and violence.
Since then, the junta has restored order and openly advocated its plan to maintain permanent influence over the society, partly through a new constitution approved by Thailand’s king last month.
The new junta-backed constitution will reportedly give the military and appointed officials a long-term grip over the country’s positions of power.
Twelve coups have been staged in Thailand since the end of direct rule by kings in 1932.