The grenade was spotted around 6.30am by a street sweeper who reported it to police.
It was found at the back of a bus stop in front of the Manulife Building. The area was temporarily sealed off while Phayathai police and explosive ordnance disposal (EOD) officers examined the scene.
Police reported the grenade's safety lever had been removed and black tape wrapped around the bomb.
EOD officers spent around 15 minutes making the grenade safe and then took it away for further inspection.
Pol Lt Athikapop Chansoda, a case investigator at Phayathai police station, said it was suspected that whoever had the grenade in their possession had becone nervous after seeing the police checkpoint at nearby Phayathai intersection, and had dumped it.
Police investigators will review CCTV footage in the area in the hope of identifying the person.