Cefazolin, also known as cefazoline and cephazolin, is a first-generation cephalosporin antibiotic used for the treatment of a number of bacterial infections. Specifically it is used to treat cellulitis, urinary tract infections, pneumonia, endocarditis, joint infection, and biliary tract infections. It is also used to prevent group B streptococcal disease around the time of delivery and before surgery. It is typically given by injection into a muscle or vein.

Common side effects include diarrhea, vomiting, yeast infections, and allergic reactions. Historically, it was thought to be contraindicated in patients with allergies to penicillin, although several recent studies have refuted this and it is proven to be safe in almost all patients, including those with known penicillin allergies. It is relatively safe for use during pregnancy and breastfeeding. Cefazolin is in the first-generation cephalosporin class of medication and works by interfering with the bacteria's cell wall.

Cefazolin was patented in 1967 and came into commercial use in 1971. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication.