The weapons were found three months ago hidden in bushes in the Prasart district after authorities learned about a recent arms delivery in the area, but the discovery was only released to the media this week, the Bangkok Post reported.
"We are trying to identify the people who hid the weapons there," Department of Special Investigations chief Tarit Pengdith said. "That's why we have only just told the media about it."
The weapons -- which included grenade launchers, rockets and rifle ammunition -- were seized in a joint operation involving the Department of Special Investigation, army rangers and the Border Patrol Police, the Post said.
Tarit said the weapons were not connected to street clashes in 2010 or to Feb. 14 explosions in the Sukhumvit area of Bangkok.