The blast that left two men dead and also one woman injured happened late Tuesday at the entrance to an office building in the centre of the capital, police said.
Kiev police spokeswoman Oksana Blyshchyk told AFP that one of the dead was a former soldier who fought against Russian-backed separatists in the country's east.
The second victim was an office guard, she added.
"Fragments found at the scene indicate that it was a hand grenade," Blyshchyk said.
Police said they were investigating the circumstances of the explosion, adding that the working version of the incident was the careless handling of an explosive device.
Attacks or accidents involving grenades and other weapons are not uncommon in Ukraine, where a large number of arms circulate due to the conflict in the east.