NYPD to Amanda Bynes -- No One Slapped Your Vagina

Amanda Bynes straight up LIED when she accused an NYPD officer of "slapping" her vagina during last week's bong arrest ... this according to cops.

After Bynes made the allegation on Twitter this weekend, the NYPD says it launched an internal investigation to see if there was any truth to Amanda's claim.

But NYPD Deputy Commissioner Paul J. Browne tells TMZ ... "Investigators have found no evidence to corroborate Ms. Bynes's allegations."

"To the contrary," Browne says ... "a credible civilian witness who was with the officers throughout told investigators that none touched Ms. Bynes inappropriately or otherwise engaged in misconduct at any time."