Microscopic polyangiitis is a pauci-immune, necrotizing, small-vessel vasculitis without evidence of granulomatous inflammation. Gastrointestinal involvement is rare and is predominantly limited to abdominal pain. Until now, the gold standard for diagnosis has been an invasive lung or kidney biopsy. We report the case of a 59-year-old woman with microscopic polyangiitis presenting as painless rectal bleeding, which was diagnosed by rectal mucosal biopsy. This is the first reported case of microscopic polyangiitis with initial presentation of rectal bleeding and diagnosis made by rectal mucosal biopsy.