The inner part of Earth’s magnetosphere contain energetic ions that carry electric currents flowing in the azimuthal direction forming a donut-shaped region called the ring current. The intensity of the currents changes with the amount of energy coming from the solar wind. Depending on the location and the strength of the ring current, the ions can excite a variety of ULF waves with the periods in the range 1-10 minutes. In this study, we report observation of a ULF wave by the Van Allen Probes spacecraft. This wave is unique in that it was excited in the plasmasphere, the region of dense background cold plasma originating from the ionosphere. In previous spacecraft studies, waves of the same type were reported only outside the plasmasphere. By combining data from multiple instruments on the spacecraft, we can determine the spatial structure of the wave and how the wave gains energy from the ring current ions. The energy exchange between the wave and ions involves drift resonance, which occurs when the wave and particle move in the azimuthal direction with the same speed.