  We show that the diagrammatic approach to quantum spin systems developed in a
seminal work by Vaks, Larkin, and Pikin [Sov. Phys. JETP 26, 188 (1968)] can be
embedded in the framework of the functional renormalization group. The crucial
insight is that the generating functional of the time-ordered connected spin
correlation functions of an arbitrary quantum spin system satisfies an exact
renormalization group flow equation which resembles the corresponding flow
equation of a system of interacting bosons. The $SU (2)$ spin algebra is
implemented via a non-trivial initial condition for the renormalization group
flow. Our method is rather general and offers a new non-perturbative approach
to quantum spin systems.
