The universal flush valve system comprises an universal system connector accommodating seating thereon of both a flapper style flush valve assembly and a canister style flush valve assembly, as required; a plurality of toilet tank specific connector extensions, each of which are threadedly engageable to a bottom portion of the universal system connector and are used to suitably engage the universal system connector to a water flow port in a toilet tank by extending through the port and receiving a suitable cooperating connection element thereover; a sealing device for sealing the required flush valve assembly against the universal system connector; and a locking mechanism for locking the required flush valve assembly onto a top portion of the universal system connector, seating over the sealing device to produce a water tight engagement therebetween.