Preoperative evaluation of the mitral valve but also of tricuspid valve and right ventricular (RV) function is mandatory in primary mitral regurgitation (MR) secondary to mitral valve prolapse (MVP). Tricuspid annulus (TA) diameter plays a pivotal role in the surgical decision to perform preventive combined tricuspid valve annuloplasty. Cardiac magnetic resonance (CMR) is the gold standard for the assessment of RV size and function. Based on 70 consecutive patients (17 women; mean age 64 ± 12) with severe MR secondary to MVP referred for CMR, we sought to assess RV geometry and function and TA dimensions and to study the interaction between TA dilatation and right-sided cardiac chambers. Frequency of RV dilatation, RV systolic dysfunction, and TA dilatation (TA diameter ≥ 40 or 21 mm/m²) were 11%, 51%, and 49%, respectively. Left ventricular (LV) end-diastolic volume index was the only independent predictor of RV dilatation. Presence of symptoms, larger LV end-diastolic volume index, and LV ejection fraction <60% were independently associated with RV dysfunction. Absolute TA diameter was 36 ± 6 mm and TA diameter index was 20 ± 3 mm/m². Reproducibility TA diameter measurement was excellent (coefficient of variation ≤10%). TR velocity >220 cm/s (odds ratio = 20.17; [3.57 to 113.90]; p = 0.001 and right atrial volume index ≥ 38 ml/m² (odds ratio = 13.44; [3.57 to 50.54]; p = 0.0001) were independent predictors of TA diameter ≥40 or 21 mm/m². CMR provides accurate right-sided cardiac chambers assessment and may help surgical planning of concomitant tricuspid valve annuloplasty before mitral valve repair in severe MR secondary to MVP. In conclusion, TA dilatation, RV enlargement, and dysfunction are related to pulmonary pressure and left-sided cardiac chambers enlargement, reflecting the long-standing consequences of severe MR.