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We present radio, optical, ultraviolet, and X-ray observations of theisolated, thermally-emitting neutron star RX J0720.4-3125 using the Parkesradio telescope, the Very Large Array, the Hubble Space Telescope, and theChandra X-ray Observatory. From these data we show that the optical/UV spectrumof RX J0720.4-3125 is not well fit by a Rayleigh-Jeans tail as previouslythought, but is instead best fit by either a single non-thermal power-law or acombination of a Rayleigh-Jeans tail and a non-thermal power-law. Takentogether with the X-ray spectrum, we find the best model for RX J0720.4-3125 tobe two blackbodies plus a power-law, with the cool blackbody implying a radiusof 11-13 km at an assumed distance of 300 pc. This is similar to many middleaged (10^{5-6} yr) radio pulsars such as PSR B0656+14, evidence supporting thehypothesis that RX J0720.4-3125 is likely to be an off-beam radio pulsar. Theradio data limit the flux at 1.4 GHz to be <0.24 mJy, or a luminosity limit of4*pi*d^2*F < 3e25*d_300^2 ergs/s, and we see no sign of extended nebulosity,consistent with expectations for a pulsar like RX J0720.4-3125.Comment: 13 pages, 9 figures. Uses emulateapj5.sty and onecolfloat5.sty.  Accepted for publication in Ap

The Nearby Neutron Star RX J0720.4−3125 from Radio to X‐Rays