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We present the discovery of the radio afterglow and near-infrared (NIR)counterpart of the Swift short GRB 200522A, located at a small projected offsetof $\approx 1$ kpc from the center of a young, star-forming host galaxy at$z=0.5536$. The radio and X-ray luminosities of the afterglow are consistentwith those of on-axis cosmological short GRBs. The NIR counterpart, revealed byour HST observations at a rest-frame time of $\approx2.3$ days, has aluminosity of $\approx (1.3-1.7) \times 10^{42}$ erg s$^{-1}$. This issubstantially lower than on-axis short GRB afterglow detections, but is afactor of $\approx 8$-$17$ more luminous than the kilonova of GW170817, andsignificantly more luminous than any kilonova candidate for which comparableobservations exist. The combination of the counterpart's color ($i-y = -0.08\pm0.21$; rest-frame) and luminosity cannot be explained by standard radioactiveheating alone. We present two scenarios to interpret the broad-band behavior ofGRB 200522A: a synchrotron forward shock with a luminous kilonova (potentiallyboosted by magnetar energy deposition), or forward and reverse shocks from a$\approx14^{\circ}$, relativistic ($\Gamma_0 \gtrsim 80$) jet. Models whichinclude a combination of enhanced radioactive heating rates, low-lanthanidemass fractions, or additional sources of heating from late-time central engineactivity may provide viable alternate explanations. If a stable magnetar wasindeed produced in GRB 200522A, we predict that late-time radio emission willbe detectable starting $\approx 0.3$-$6$ years after the burst for a depositedenergy of $\approx 10^{53}$ erg. Counterparts of similar luminosity to GRB200522A associated with gravitational wave events will be detectable withcurrent optical searches to $\approx\!250$ Mpc.

The Broad-band Counterpart of the Short GRB 200522A at $z=0.5536$: A Luminous Kilonova or a Collimated Outflow with a Reverse Shock?