Sialadenitis has rarely been reported in patients with infectious mononucleosis (IM). Our patient was a 22-year-old man who presented with bilateral swelling of the parotid and submandibular glands, a fever, malaise, and splenomegaly. Laboratory tests revealed an increased percentage of atypical lymphocytes in the leukocyte fraction. Serological testing for antibodies against Epstein-Barr virus (EBV) revealed an acute infection pattern. The patient was diagnosed with sialadenitis associated with IM caused by EBV infection. With symptomatic treatment, the salivary gland swelling completely resolved within a week. This case suggests that EBV-induced IM should be included in the differential diagnosis of diffuse sialadenitis with elevated atypical lymphocyte counts.