Investigators found remnants of batteries and wires at the scene of the blast on the ground floor of Phramongkutklao Hospital, said Srivara Ransibrahmanakul, the deputy national police chief.
"We can confirm at this stage that it is a bomb," he said.
Three people suffered serious shrapnel injuries to the face and neck, but most of the victims had minor wounds, said Sansern Kaewkumnerd, a government spokesman.
It was not immediately clear who was behind the explosion or if it was connected to the anniversary of the 2014 military coup that overthrew a democratically elected government.
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Phramongkutklao is a military-run hospital that is also open to civilians. The blast radius from the explosion was 2 to 3 meters (up to 9 feet), police said.
Since the 2014 coup, at least six explosions have occurred in Bangkok. Last Monday, a bomb went off in front of the country's National Theater. Last month, a similar explosion took place in front of an old government lottery office.
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