Thailand remains starkly divided since the May 22, 2014 coup, but dissent has broadly been silenced by a military with sweeping security powers.
It was not immediately clear who was behind the blast but the country has a long history of small bombs being used by various political and militant groups, especially during significant anniversaries.
Police investigations rarely get to the bottom of who is culpable for the attacks.
"From the initial report it was a bomb... There are more than 20 people injured," Deputy National Police Chief General Srivara Rangsibrahmanakul told reporters.
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"It was likely to be in a package," Srivara said, adding that "battery and wires" were found at the scene.
The clinic in central Bangkok -- King Mongkut Hospital -- is often used by serving and retired members of Thailand's armed forces.
Police said the hospital was not evacuated while bomb disposal officers scoured the scene.
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.
Last Monday, a small pipe bomb detonated outside Bangkok's National Theatre, close to the Grand Palace.
On a subsequent walkabout Bangkok's city police chief declared the explosion was caused by a broken water pipe, only to later backtrack when other officials briefed it was a small homemade bomb.
Since the coup the political violence that defined much of the last decade had subsided, although peace has largely been achieved through repression.
Militant elements among pro-democracy groups have either been arrested or have gone to ground.
Over the past 10 years Thais have witnessed repeated rounds of deadly protests, a string of short-lived governments and two military coups that deposed elected leaders.
The junta says its 2014 coup -- the 12th time generals have successfully seized power -- was needed to bring stability and root out corruption.