So far, organic semiconductors have been mainly proposed as detectors for ionizing radiation in the indirect conversion approach, i.e. as scintillators, which convert ionizing radiation into visible photons, or as photodiodes, which detect visible photons coming from a scintillator and convert them into an electrical signal. The direct conversion of ionizing radiation into an electrical signal within the same device is a more effective process than indirect conversion, since it improves the signal-to-noise ratio and it reduces the device response time. We report here the use of Organic Semiconducting Single Crystals (OSSCs) as intrinsic direct ionizing radiation detectors, thanks to their stability, good transport properties and large interaction volume. Ionizing radiation X-ray detectors, based on low-cost solution-grown OSSCs, are here shown to operate at room temperature, providing a stable linear response with increasing dose rate in the ambient atmosphere and in high radiation environments.