We observed the � -ray pulsar Geminga with the FUV-MAMA and NUV-MAMA detectors of the Space Telescope Imaging Spectrometer to measure Geminga’s spectrum and pulsations in the ultraviolet. The slope of the far-ultraviolet (FUV) spectrum is close to that of a Rayleigh-Jeans spectrum, suggesting that the FUV radiation is dominated by thermal emissionfromthe neutron star(NS) surface.ThemeasuredFUVflux,FFUV ¼ (3:7 � 0:2) ; 10 � 15 ergscm � 2 s � 1 inthe1155–17028band,correspondstoabrightnesstemperatureTRJ � (0:3–0:4)(d200/R13) 2 MK,dependingon the interstellarextinction(d ¼ 200d200 pcandR ¼ 13R13 kmarethedistanceandtheNSradius,respectively).Thesoft thermal component of Geminga’s X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature Ts ¼ 0:49 � 0:01 MK and radius Rs ¼ (12:9 � 1:0)d200 km. Contrary to other NSs detected in the UV-optical,forwhich theextrapolation ofthe X-ray thermalcomponentinto theopticalunderpredicts the observed flux of thermal radiation, the FUV spectrum of Geminga lies slightly below the extrapolation of the soft thermal component,which might beassociatedwith Geminga’s very low temperature.Surprisingly, thethermalFUVradiation is strongly pulsed, showing a narrow dip at a phase close to that of a broader minimum of the soft X-ray light curve. The strong pulsations might be attributed to partial occultations of the thermal UV radiation by regions of the magnetosphere filled with electron/positron plasma. In contrast to the FUV spectrum, the near-infrared (NIR) through near-ultraviolet (NUV) spectrum of Geminga is clearly nonthermal. It can be described by a power-law model, F� / � � � þ1 , with a photon index � ¼ 1:43 � 0:15, close to the slope � ¼ 1:56 � 0:24 of the hard X-ray (E > 2:5 keV) magnetospheric component. The extrapolation of the X-ray magnetospheric spectrum into the optical is marginally consistent with (or perhaps lies slightly above) the observed NIR-optical-NUV spectrum. The NUV pulsations, however, do not show a clear correlation with the hard X-ray pulsations. Subject headingg pulsars: individual (Geminga) — stars: neutron — ultraviolet: stars