Background: Left (LV) and right ventricular (RV) function are proposed to be intimately linked. We previously demonstrated that RV dysfunction increases with severity of LV dysfunction. Atrial fibrillation (AF) is an established cause of LV systolic dysfunction, but its effect on RV size and function in this population is not well characterised. We aim to evaluate the effect of AF on RV systolic function in a cohort of non-ischaemic cardiomyopathy patients. Methods: We compared 64 consecutive patients in sinus rhythm (SR) with 32 patients in AF with stable HFrEF in the absence of coronary artery, significant valvular, congenital, or pulmonary disease. Patients in both SR and AF were categorised based on LVEF (mild 40–49%, moderate 30–39%, severe <30%). Standard and novel echocardiographic parameters of right heart were assessed. RV free wall peak systolic strain (RVFWS) was measured (TomTec Image Arena, Germany v4.6). Results: Patients with SR (mean age 56 ± 19 y; 61% male), and AF (mean age 67 ± 11 y; 75% male) groups had similar proportions of patients in the mild, moderate and severe groups (SR: n = 25, 21, 18 vs AF n = 10, 12, 10, respectively). Conditional regression analysis revealed that patients with AF had significantly worse RV impairment compared to patients with SR as defined by RV fractional area change (RVFAC; p < 0.001) and RVFWS (p < 0.001) across all grades of LV impairment. Conclusions: Our results demonstrate that AF is associated with worse RV impairment compared to patients in SR as defined by RVFAC and RVFWS. This highlights the potential benefits of restoration of SR in this cohort of HFrEF patients.