Thermomechanical fatigue (TMF) and isothermal fatigue (IF) of polycrystalline nickel-based superalloy C263 were studied. Strain-controlled fatigue experiments were conducted under in-phase and out-of-phase TMF modes in the temperature range of 300-800°C, and under high-temperature IF conditions at 800°C. The cycle duration using a triangular shape form was 100 s. The cooling/heating rate of the TMF cycle was 10°C/s. A thorough analysis of hysteresis loops was performed, and cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained. The presence of dynamic strain ageing and thermal recovery on the stress-strain response during TMF tests was observed. To gain insights into how temperature and strain affect the material, scanning electron microscopy and transmission electron microscopy techniques were utilised. Investigation revealed that in-phase TMF loading promoted internal intergranular damage and thus provided the lowest fatigue lifetimes across all selected strain amplitudes out of all three testing conditions. In contrast, out-of-phase TMF loading induced higher stresses and promoted transgranular fatigue crack propagation through a striation mechanism, resulting in longer fatigue lifetimes during TMF conditions. The microstructural observations revealed the occurrence of dynamic recrystallisation during TMF and IF. Fine recrystallised grains formed most prominently during TMF-OP, while TMF-IP and IF showed only localised recrystallisation near secondary cracks. This observation demonstrates that recrystallisation can be activated during cyclic TMF loading, where it influences crack propagation behaviour and can potentially reduce the lifetime of C263.