Psychosis may involve delusional beliefs. A delusion is a fixed, false idiosyncratic belief, which does not change even when presented with incontrovertible evidence to the contrary. Delusions are context- and culture-dependent: a belief which inhibits critical functioning and is widely considered delusional in one population may be common (and even adaptive) in another, or in the same population at a later time. Since normative views may contradict available evidence, a belief need not contravene cultural standards in order to be considered delusional.

Prevalence in schizophrenia is generally considered at least 90%, and around 50% in bipolar disorder.

The DSM-5 characterizes certain delusions as "bizarre" if they are clearly implausible, or are incompatible with the surrounding cultural context. The concept of bizarre delusions has many criticisms, the most prominent being judging its presence is not highly reliable even among trained individuals.

A delusion may involve diverse thematic content. The most common type is a persecutory delusion, in which a person believes that an entity seeks to harm them. Others include delusions of reference (the belief that some element of one's experience represents a deliberate and specific act by or message from some other entity), delusions of grandeur (the belief that one possesses special power or influence beyond one's actual limits), thought broadcasting (the belief that one's thoughts are audible) and thought insertion (the belief that one's thoughts are not one's own). A delusion may also involve misidentification of objects, persons, or environs that the afflicted should reasonably be able to recognize; such examples include Cotard's syndrome (the belief that oneself is partly or wholly dead) and clinical lycanthropy (the belief that oneself is or has transformed into an animal).

The subject matter of delusions seems to reflect the current culture in a particular time and location. For example, in the US, during the early 1900s syphilis was a common topic, during the Second World War Germany, during the Cold War communists, and in recent years, technology has been a focus. Some psychologists, such as those who practice the Open Dialogue method, believe that the content of psychosis represents an underlying thought process that may, in part, be responsible for psychosis, though the accepted medical position is that psychosis is due to a brain disorder.

Historically, Karl Jaspers classified psychotic delusions into primary and secondary types. Primary delusions are defined as arising suddenly and not being comprehensible in terms of normal mental processes, whereas secondary delusions are typically understood as being influenced by the person's background or current situation (e.g., ethnicity; also religious, superstitious, or political beliefs).