A tearaway introducer sheath assembly (100,200) having an integrated valve (300). The valve is seated within a valve housing (222) defined by the hub portion (200) of the sheath (200). The valve (300) includes a pair of distally extending opposed side walls or flaps (324) extending to converge at a distal tip (328) having a virtual opening (334) therethrough. The valve further includes a pair of tensioners (340) along outer surfaces of the opposed side walls (324) that extend radially outwardly to engage and bear against the interior valve housing surface (230) to press the opposed side walls (324) together at the distal tip (328) for sealing, both when a dilator extends through the valve and sheath and afterward upon removal of the dilator.