Selena Gomez to Justin Bieber -- Cry YOU a River? Cry ME a River!

Justin Bieber isn't the only one who can belt out Justin Timberlake's classic "Cry Me a River" ... turns out his currently ex-girlfriend, Selena Gomez, has a version of her own up her sleeve.

You may remember ... back in November, after one of their numerous splits, Justin performed the JT track on stage in Boston.

Last night in New York, it was Selena's turn ... doing her own rendition at a show for UNICEF in NYC.

So we gotta ask ...