Primum non nocere (Classical Latin: [ˈpriːmũː noːn nɔˈkeːrɛ]) is a Latin phrase that means "first, do no harm". The phrase is sometimes recorded as primum nil nocere.

Non-maleficence, which is derived from the maxim, is one of the principal precepts of bioethics that all students in healthcare are taught in school and is a fundamental principle throughout the world. Another way to state it is that, "given an existing problem, it may be better not to do something, or even to do nothing, than to risk causing more harm than good." It reminds healthcare personnel to consider the possible harm that any intervention might do. It is invoked when debating the use of an intervention that carries an obvious risk of harm but a less certain chance of benefit.

Non-maleficence is often contrasted with its corollary, beneficence.