A radiation imaging apparatus includes a pixel array including a plurality of pixels, a readout circuit configured to read out signals from the pixel array, a detection unit configured to detect, based on radiation emitted from a radiation source or information provided from the radiation source, a start of irradiation of radiation by the radiation source, and a control unit configured to determine a timing of each of a plurality of sample and hold operations in each of the plurality of pixels each time the start of irradiation of radiation is detected by the detection unit. The timing of at least one sample and hold operation of the plurality of sample and hold operations is a timing in a radiation irradiation period, and each of the plurality of pixels includes a conversion element configured to convert radiation into an electrical signal, and a sample and hold circuit configured to sample and hold the signal from the conversion element over a plurality of times in accordance with the timing of each of the plurality of sample and hold operations determined by the control unit.