Coherent spin resonance methods such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy have led to spectrally highly sensitive, non-invasive quantum imaging techniques with groundbreaking applications in fields such as medicine, biology, and physics. Meanwhile, transmission electron microscopy (TEM) offers detailed investigations with sub-atomic resolution, but often inflicts significant radiation damage. Here we exploit synergies and report on an integration of ESR spectroscopy in a TEM. Our miniaturized ESR setup, optimized for microscopic sample sizes, is implemented on a standard TEM sample holder and leverages the strong magnetic field of the TEM polepiece to align and energetically separate spin states. This integration will facilitate in situ studies of spin systems and their dynamics, quantum materials, radicals, electrochemical reactions, and radiation damage — properties that have, until now, been difficult to access using conventional electron microscopic tools. Moreover, this development marks a significant technological advancement towards microwave-driven quantum spin studies with a highly controlled electron probe at the nanoscale.