Outpatient surgery, also known as ambulatory surgery, day surgery, day case surgery, or same-day surgery, is surgery that does not require an overnight hospital stay. The term “outpatient” arises from the fact that surgery patients may enter and leave the facility on the same day. The advantages of outpatient surgery over inpatient surgery include greater convenience and reduced costs.^: 24–26

Outpatient surgery may occur in an inpatient facility, in a self-contained unit within a hospital (also known as a hospital outpatient department), in a freestanding self-contained unit (also known as an ambulatory surgery center), or in a physician's office-based unit.^: 61 Between the late 20th century and early 21st century, outpatient surgery has grown in popularity in many countries.^: 22 In the United States, 65% of surgeries at hospitals in 2012 were conducted on an outpatient basis, compared with 54% in 1992.

Studies have shown that outpatient surgery is as safe as or safer than inpatient surgery. For instance, complication rates and post-surgical hospitalization or readmission rates are comparable, and pain and infection rates are lower after outpatient surgery than inpatient surgery.^: 24 Nevertheless, articles in the newsmedia (such as some discussing the 2014 death of Joan Rivers after an outpatient procedure) have questioned the safety of outpatient surgery performed at ambulatory surgery centers.