The ferritic/martensitic steel HT9 was irradiated in the BOR-60 reactor at 650, 690 and 730 K (377, 417 and 457 °C) to doses between ∼14.6–18.6 displacements per atom (dpa). Irradiated samples were comprehensively characterized using analytical scanning/transmission electron microscopy and atom probe tomography, with emphasis on the influence of irradiation temperature on microstructure evolution. Mn/Ni/Si-rich (G-phase) and Cr-rich ($\alpha$ʹ) precipitates were observed within martensitic laths and at various defect sinks at 650 and 690 K (377 and 417 °C). For both G-phase and $\alpha$ʹ precipitates, the number density decreased while the size increased with increasing temperature. At 730 K (457 °C), within martensitic laths, a very low density of large G-phase precipitates nucleating presumably on dislocation lines was observed. No $\alpha$ʹ precipitates were observed at this temperature. Both a \textless100\textgreater and a/2 \textless111\textgreater type dislocation loops were observed, with the a \textless100\textgreater type being the predominant type at 650 and 690 K (377 and 417 °C). On the contrary, very few dislocation loops were observed at 730 K (457 °C), and the microstructure was dominated by a/2 \textless111\textgreater type dislocation lines (i.e., dislocation network) at this temperature. Small cavities (diameter \textless 2 nm) were observed at all three temperatures, whereas large cavities (diameter \textgreater 2 nm) were observed only at 690 K (417 °C), resulting in a bimodal cavity size distribution at 690 K (417 °C) and a unimodal size distribution at 650 and 730 K (377 and 457 °C). The highest swelling (%) was observed at 690 K (417 °C), indicating that the peak of swelling happens between 650 and 730 K (377 and 457 °C).