Previous research shows the beneficial effects of an intradialytic resistance training (IRT) on muscle function in haemodialysis patients. However, patients vary highly in their functional responses to IRT, may be due to effects of age and sex heterogeneities in adaptation. Therefore, the aim of this study was to investigate the degree to which the effects of IRT on the muscle function of haemodialysis patients vary by age and sex. We included 57 patients who completed a 12-week IRT (EXG) and 33 patients who received no IRT (CNG) during haemodialysis. Muscle function (MF) was assessed using dynamometry before and after a 12-week intervention and after a 12-week follow-up. After the 12-week intervention, we found a moderation effect of age in the relative (%) change (p = 0.011) and absolute (Δ) change (p = 0.027) of MF, and a moderation effect of sex in %MF (p = 0.001), but not in ΔMF (p = 0.069). Regarding patients’ age, the change of MF was only significantly different between EXG and CNG patients aged 60–70 years (%MF, EXG: + 34.6%, CNG: − 20.1%, p < 0.001; ΔMF, EXG: + 44.4 N, CNG: − 22.1 N, p < 0.001). Regarding patients’ sex, the change of MF was only significantly different between EXG and CNG female patients (%MF, EXG: + 23.9%, CNG: − 23.6%, p < 0.001). Age and sex did not significantly moderate changes in MF measures after 12 weeks of follow-up. We conclude that both age and sex of haemodialysis patients affect their functional response to IRT in the short term. Trial Registration: Intradialytic Resistance Training in Haemodialysis Patients (IRTHEP)—#NCT03511924, 30/04/2018, https://clinicaltrials.gov/ct2/show/NCT03511924.