Trimethoprim/sulfamethoxazole, (also known as Cotrimoxazole) sold under the brand name Bactrim among others, is a fixed-dose combination antibiotic medication used to treat a variety of bacterial infections. It consists of one part trimethoprim to five parts sulfamethoxazole. It is used to treat urinary tract infections, methicillin-resistant Staphylococcus aureus (MRSA) skin infections, travelers' diarrhea, respiratory tract infections, and cholera, among others. It is used both to treat and prevent pneumocystis pneumonia and toxoplasmosis in people with HIV/AIDS and other causes of immunosuppression. It can be given by mouth or intravenously.

Trimethoprim/sulfamethoxazole is on the World Health Organization's List of Essential Medicines and is also available as a generic medication. In 2020, it was the 121st most commonly prescribed medication in the United States, with more than 5 million prescriptions.