  Giant radio pulses (GRPs) are sporadic bursts emitted by some pulsars,
lasting a few microseconds. GRPs are hundreds to thousands of times brighter
than regular pulses from these sources. The only GRP-associated emission
outside radio wavelengths is from the Crab Pulsar, where optical emission is
enhanced by a few percent during GRPs. We observed the Crab Pulsar
simultaneously at X-ray and radio wavelengths, finding enhancement of the X-ray
emission by $3.8\pm0.7\%$ (a 5.4$\sigma$ 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.
