We present the first spatially and spectrally resolved image of the molecular outflow in the western nucleus of Arp 220. The outflow, seen in HCN (1-0) by the Atacama Large millimeter/sub-millimeter Array, is compact and collimated, with an extension 120 pc. Bipolar morphology emerges along the minor axis of the disk, with redshifted and blueshifted components reaching a maximum inclination-corrected velocity of km s−1. The outflow is also seen in CO and continuum emission, the latter implying that it carries significant dust. We estimate a total mass in the outflow of , a dynamical time of ∼105 yr, and mass outflow rates of yr−1 and yr−1 for the northern and southern lobes, respectively. Possible driving mechanisms include supernovae energy and momentum transfer, radiation pressure feedback, and a central AGN. The latter could explain the collimated morphology of the HCN outflow; however, we need more complex theoretical models, including contributions from supernovae and AGN, to pinpoint the driving mechanism of this outflow.