Rationalization is a defense mechanism (ego defense) in which apparent logical reasons are given to justify behavior that is motivated by unconscious instinctual impulses. It is an attempt to find reasons for behaviors, especially one's own. Rationalizations are used to defend against feelings of guilt, maintain self-respect, and protect oneself from criticism.

Rationalization happens in two steps:

1. A decision, action, judgement is made for a given reason, or no (known) reason at all.
2. A rationalization is performed, constructing a seemingly good or logical reason, as an attempt to justify the act after the fact (for oneself or others).

Rationalization encourages irrational or unacceptable behavior, motives, or feelings and often involves ad hoc hypothesizing. This process ranges from fully conscious (e.g. to present an external defense against ridicule from others) to mostly unconscious (e.g. to create a block against internal feelings of guilt or shame). People rationalize for various reasons—sometimes when we think we know ourselves better than we do. Rationalization may differentiate the original deterministic explanation of the behavior or feeling in question.

Many conclusions individuals come to do not fall under the definition of rationalization as the term is denoted above.