The World Health Organization estimates that there is greater than one million new cases of sexually transmitted infections (STIs) every day. In many countries, STIs are at an unprecedented high, including the USA, where nearly 20 million new cases were reported in 2016. Although morbidity associated with STIs is usually seen in the context of genitourinary disease, these pathogens may also affect the gastrointestinal tract and cause anal pain, abdominal pain, or diarrhea. It is important to recognize patterns of injury associated with these pathogens, especially those that may mimic other gastrointestinal diseases, such as idiopathic inflammatory bowel disease (IBD). This review focuses upon STIs of the lower gastrointestinal tract, organized by the most common site of involvement: the anus, rectum, and colon.