Cellulitis is a potentially serious bacterial skin infection. Penile cellulitis refers to the inflammation of the penile shaft and commonly occurs in uncircumcised, sexually active young adults. We reported the case of a 25-year-old heterosexual circumcised male patient with a two-day history of swelling and pain over the penile shaft. Local examination revealed a diffusely swollen penile shaft, erythematous, warm to the touch, and tender. The penile discharge culture was suggestive of a Streptococcus species infection. Ultrasonography of the penis showed increased echogenicity of the left side of the penile shaft soft tissue with a markedly increased Doppler signal, indicating cellulitis. Based on these findings, the patient was diagnosed with cellulitis of the penis and managed with broad-spectrum antibiotics. Though history and clinical examination are sufficient to diagnose penile cellulitis, our case highlighted that ultrasound could also support the diagnosis of penile cellulitis and help rule out differentials.