Hindsight bias, also known as the knew-it-all-along phenomenon or creeping determinism, is the common tendency for people to perceive past events as having been more predictable than they were.

People often believe that after an event has occurred, they would have predicted or perhaps even would have known with a high degree of certainty what the outcome of the event would have been before the event occurred. Hindsight bias may cause distortions of memories of what was known or believed before an event occurred, and is a significant source of overconfidence regarding an individual's ability to predict the outcomes of future events. Examples of hindsight bias can be seen in the writings of historians describing outcomes of battles, physicians recalling clinical trials, and in judicial systems as individuals attribute responsibility on the basis of the supposed predictability of accidents.

In some countries, 20/20 indicates normal visual acuity at 20 feet, from which derives the idiom "hindsight is 20/20".