Swollen buttocks on the left side of the anus may be caused by perianal abscesses, boils, and muscle strains. 1. Perianal abscess: when the perianal tissue is infected by pathogens such as bacteria, it can spread to the loose fatty connective tissue of the perirectal-anal canal space, which may lead to anal left buttock swelling. 2. Boils: If the folliculitis near the anus is infected by pyogenic bacteria such as staphylococcus, it may cause a painful left buttock swelling. 3. Muscle strain: Muscle strain due to strenuous activities, so that the local soft tissues are damaged, the phenomenon of left buttock swelling may occur. The phenomenon of anal left buttocks swelling, it is recommended to go to the hospital in time, clear cause, under the guidance of the doctor for standardized treatment, so as not to delay the condition.