AbstractWe present the first case in the literature of a 78‐year‐old woman with recurrent cardiac sarcoma adjacent to a bioprosthetic mitral valve treated with magnetic resonance linear accelerator (MR‐Linac) guided adaptive stereotactic ablative body radiotherapy (SABR). The patient was treated using a 1.5 T Unity MR‐Linac system (Elekta AB, Stockholm, Sweden). The mean gross tumour volume (GTV) size was 17.9 cm3(range 16.6–18.9 cm3) based on daily contours and the mean dose received by the GTV was 41.4 Gy (range 40.9–41.6 Gy) in five fractions. All fractions were completed as planned and the patient tolerated the treatment well with no acute toxicity reported. Follow‐up appointments at 2 and 5 months after the last treatment showed stable disease and good symptomatic relief. Results of transthoracic echocardiogram after radiotherapy showed that the mitral valve prosthesis was normally seated with regular functionality. This study provides evidence that MR‐Linac guided adaptive SABR is a safe and viable option for the treatment of recurrent cardiac sarcoma with mitral valve bioprosthesis.