Chlamydia trachomatis-lymphogranuloma venereum (LGV) is a sexually transmitted infection (STI) and an uncommon cause of proctitis and rectal mass. When the rectum is affected, patients may experience symptoms such as rectal bleeding, pain, and tenesmus, which can resemble proctitis or proctocolitis and may be mistaken for malignancy. We present the case of a 40-year-old male patient with well-controlled HIV, who initially exhibited symptoms and imaging findings concerning for rectal neoplasia. Following colonoscopy with biopsy and a rectal swab polymerase chain reaction (PCR), Chlamydia trachomatis was identified as the cause. This case emphasizes the importance of considering LGV proctitis when forming a differential for rectal masses, highlighting its potential to mimic malignancy and the importance of accurate diagnosis and prompt treatment.