The occurrence of vaginal calculi is rare in animals. This report describes the first record of a vaginal calculus in an adult captive red-rumped agouti (Dasyprocta leporina). The specimen presented with a swelling in the genital region, which was firm and mobile on palpation. Direct examination revealed a calculus occupying the entire vaginal cavity. Surgical intervention consisted of a ventral vulvar incision and direct removal of the calculus, followed by appropriate suturing of the musculature, subcutaneous tissue and skin. The agouti showed good clinical recovery, with proper wound healing and return to normal physiological conditions. Post-operative abdominal ultrasonography ruled out urinary tract alterations, suggesting an exclusively vaginal origin of the calculus. Chemical analysis of the calculus revealed a mixed composition of calcium carbonate, calcium oxalate, calcium phosphate (hydroxyapatite) and ammonium–magnesium phosphate (struvite). This report highlights the importance of recognizing the occurrence of vaginal calculi in agoutis and of including them in the differential diagnosis of clinical signs such as urinary incontinence, pollakiuria, haematuria and genital swelling.