Cytomegalovirus (CMV) infection has been associated with ulcerative colitis (UC). The prevalence of CMV infection in UC patients ranges from 10% to 16%. It is particularly high in the patients with steroid-refractory UC and those treated with immunosuppressants. However, synchronous onset of CMV colitis and UC in an immunocompetent patient is rare. It was originally described in 1990 and since then sixteen cases have been reported, as far as we are aware. Here, we present a case of CMV colitis and UC synchronously developing in an elderly immunocompetent woman. She was diagnosed through tissue immunohistochemistry and successfully treated with intravenous ganciclovir. This case demonstrates that in patients with severe active UC, even with new onset of the disease, CMV infection needs to be ruled out before initiating an aggressive immunosuppressive therapy.