Surprise (sə(r)ˈprīz^ⓘ) is a rapid, fleeting, mental and physiological state. It is related to the startle response experienced by animals and humans as the result of an unexpected event.

Surprise can have any valence. That is, it can be pleasant/positive, unpleasant/negative, or neutral/moderate. Surprise can occur in varying levels of intensity ranging from very surprised, which may induce the fight-or-flight response, or slightly surprised, which elicits a less intense response to the stimulus.

Surprise is included as a primary or basic emotion in the taxonomies of Carroll Izard and Paul Ekman. According to these perspectives, surprise is evolutionarily adaptive, and also innate and universal across human cultures.

Surprise is intimately connected to the idea of acting in accordance with a set of rules. When the rules of reality generating events of daily life separate from the rule-of-thumb expectations, surprise is the outcome. Surprise represents the difference between expectations and reality, the gap between our assumptions and expectations about worldly events and the way that those events actually turn out. This gap can be deemed an important foundation on which new findings are based since surprises can make people aware of their own ignorance. The acknowledgement of ignorance, in turn, can mean a window to new knowledge.

Surprise can also occur due to a violation of expectancies. In the specific case of interpersonal communication, the Expectancy Violation Theory (EVT) says that three factors influence a person's expectations: interactant variables, environmental variables, and variables related to the nature of the interaction or environmental variables.

Surprise may occur due to a violation of one, two, or a combination of all three factors.

Surprise does not always have to have a negative valence. EVT proposes that expectancy's will influence the outcome of the communication as a confirmation, behaviors within the expected range, or violation, behaviors outside the expected range. EVT also postulates that positive interactions will increase the level of attraction of the violator, whereas negative violations decrease the attraction. Positive violations would then cause positive surprise, such as a surprise birthday party, and negative violations would cause negative surprise, such as a parking ticket. Positive violations may enhance credibility, power, attraction, and persuasiveness. Negative violations may reduce them.

The physiological response of surprise falls under the category of the startle response. The main function of surprise or the startle response is to interrupt an ongoing action and reorient attention to a new, possibly significant event. There is an automatic redirection of focus to the new stimuli and, for a brief moment, this causes tenseness in the muscles, especially the neck muscles. Studies show that this response happens extremely fast, with information (in this case a loud noise) reaching the pons within 3 to 8 ms and the full startle reflex occurring in less than two tenths of a second.