The explosive device is believed to have been fired with an M79 grenade launcher.
It hit a house next to the Democrat Party HQ compound in Setthasiri Soi 2 in the Samsen Nai area at 7.30pm.
The blast damaged a BMW and Honda Jazz parked on the premises, around 20 metres from the main parking area of the Democrat HQ building.
It is believed the attacker fired the grenade from the nearby local road and was targeting the Democrat compound but missed.
In an earlier attack on Jan 13, unknown gunmen in a Toyota sedan fired eight 9mm rounds at Democrat Party HQ at 2.30am. The shots damaged the party's coffee house, which fronts the building. No one was injured.