In jurisprudence, diminished responsibility (or diminished capacity) is a defense by excuse via which a defendant argues that that although they broke the law, they should not be held criminally liable for doing so, as their mental functions were "diminished" or impaired.

This is similar to an insanity defense. Peter Arenella, in the Columbia Law Review (1977 p.830), stated, "the defense [of diminished responsibility]...was first recognized by Scottish common law to reduce the punishment of the 'partially insane'." An example of a "diminished capacity" might be extremely low intelligence.

This defense usually does not necessarily result in a verdict of "not guilty"; it often results in the substitution of a lesser offence (eg manslaughter instead of murder), or a mitigated sentence.

The California Penal Code states (2002), "The defense of diminished capacity is hereby abolished ... there shall be no defense of diminished capacity, diminished responsibility, or irresistible impulse..."