We present deep new Gran Telescopio Canarias (GTC) narrow-band images and William Herschel Telescope (WHT) long-slit spectroscopy of the merging system Mrk273 that show a spectacular extended halo of warm ionized gas out to a radius of ˜45 kpc from the system nucleus. Outside of the immediate nuclear regions (r > 6 kpc), there is no evidence for kinematic disturbance in the ionized gas: in the extended regions covered by our spectroscopic slits the emission lines are relatively narrow (full width at half-maximum, FWHM ≲ 350 km s-1) and velocity shifts small (|ΔV| ≲ 250 km s-1). This is despite the presence of powerful near-nuclear outflows (FWHM > 1000 km s-1; |ΔV| > 400 km s-1; r < 6 kpc). Diagnostic ratio plots are fully consistent with Seyfert 2 photoionization to the NE of the nuclear region, however to the SW the plots are more consistent with low-velocity radiative shock models. The kinematics of the ionized gas, combined with the fact that the main structures are aligned with low-surface-brightness tidal continuum features, are consistent with the idea that the ionized halo represents tidal debris left over from a possible triple-merger event, rather than a reservoir of outflowing gas.