Blabbermouth

Noun

 * 1) a gossip
 * If it weren't for that blabbermouth Sally, the principal would never have known we did it.

Thesaurus
babbler, betrayer, blab, blabber, blabberer, chatterer, delator, fink, gabber, gossip, informer, jabberer, magpie, narc, peacher, prater, prattler, snitch, snitcher, spy, squealer, stool pigeon, stoolie, talebearer, tattler, tattletale, telltale, whistle-blower