Pulsars are spinning, magnetized neutron stars that are observed as a regular sequence of radio pulses. Most pulses are of consistent intensity, but occasionally one is brighter by orders of magnitude. The cause of these unpredictable giant radio pulses (GRPs) is unknown. Enoto et al. observed the Crab Pulsar simultaneously with x-ray and radio telescopes. They found that x-ray emission during GRPs was slightly brighter than that during normal pulses. Comparing the radio and x-ray enhancements provides constraints on the GRP emission mechanism and the possible connections with other transient radio phenomena.Science, this issue p. 187Giant radio pulses (GRPs) are sporadic bursts emitted by some pulsars that last a few microseconds and are hundreds to thousands of times brighter than regular pulses from these sources. The only GRP-associated emission outside of radio wavelengths is from the Crab Pulsar, where optical emission is enhanced by a few percentage points during GRPs. We observed the Crab Pulsar simultaneously at x-ray and radio wavelengths, finding enhancement of the x-ray emission by 3.8 textpm 0.7a 5.4σ detection) coinciding with GRPs. This implies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known. We discuss the implications for the pulsar emission mechanism and extragalactic fast radio bursts.