  We calculate, for the first time, $J/\psi$ triangular flow in high energy
nuclear collisions. The charmonium motion in medium is controlled by a
transport equation with loss and gain terms, and the evolution of the hot
medium is governed by a single-shot hydrodynamic equation with a triangularly
deformed initial condition. In comparison with the elliptic flow $v_2$, the
triangular flow $v_3$ comes almost completely from the charmonium regeneration
in the quark gluon plasma and therefore is more sensitive to the medium
properties.
