We propose an alternative algebraic semantics for modal logic, using quantales. To this end, we start by introducing the notion of d-quantale, based on which we obtain a conservative extension of the traditional algebraic semantics of modal logic. This extension leads, in particular, to a generalization of the correspondence between Kripke structures and modal algebras, in a way that is useful for instance in understanding the specification of transition systems by means of branching temporal logic.