In law, good faith (in Latin, bona fides) is the mental and moral state of honest, even if objectively unfounded, conviction as to the truth or falsehood of a proposition or body of opinion, or as to the rectitude or depravity of a line of conduct.

One who acts in good faith, so far as the violation of positive law (or even in certain junctures of natural law) is concerned, is said to labor under an invincible error, and hence to be guiltless. This consideration is frequently applied to determine the degree of right or obligation prevailing in the various forms of human engagements, such as contracts and the law of obligations.

In the matter of prescription, good faith is held to be an indispensable requirement whether there be question of acquiring dominion or freeing oneself from a burden. Also, in deciding the duty incumbent upon one who finds himself in possession of another's property, cognizance is taken of the good faith with which perchance the holding began and was accompanied.