Ionizing radiation is a well-appreciated health risk, precipitant of DNA damage, and contributes to DNA methylation variability. Nevertheless, relationships of ionizing radiation with DNA methylation-based markers of biological age (i.e. epigenetic clocks) remain poorly understood. Using existing data from human bronchial epithelial cells, we examined in vitro relationships of three epigenetic clock measures (Horvath DNAmAge, MiAge, and epiTOC2) with galactic cosmic radiation (GCR), which is particularly hazardous due to its high linear energy transfer (LET) heavy ion components. High-LET 56Fe was significantly associated with accelerations in epiTOC2 (β=192 cell divisions, 95%CI: 71, 313, P-Value=0.003). We also observed a significant, positive interaction of 56Fe ions and time-in-culture with epiTOC2 (95%CI: 42, 441, P-Value=0.019). However, only the direct 56Fe ion association remained statistically significant after adjusting for multiple hypothesis testing. Epigenetic clocks were not significantly associated with high-LET 28Si and low-LET xrays. Our results demonstrate sensitivities of specific epigenetic clock measures to certain forms of GCR. These findings suggest that epigenetic clocks may have some utility for monitoring and better understanding the health impacts of GCR.