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

Diminished capacity is a partial defense to charges that require that the defendant act with a particular state of mind. For example, if the felony murder rule does not apply, first degree murder requires that the state prove beyond a reasonable doubt that the defendant acted with premeditation, deliberation, and the specific intent to kill—all three are necessary elements of the state's case. If evidence exists, sufficient to create a reasonable doubt as to whether the defendant because of mental illness or "defect" possessed the capacity to premeditate, deliberate or form the specific intent to kill then the state cannot convict the defendant of first degree murder. This does not mean that the defendant is entitled to an acquittal. The defendant still might be convicted of second-degree murder which only requires that the defendant act with general malice.

The defense's acceptance in American jurisdictions varies considerably. The majority of states have adopted it by statute or case decision, and a minority even recognise broader defenses such as "irresistible impulse". Some U.S. states restrict the defense to the charge of murder only where a successful defense will result in a manslaughter conviction instead of murder. Until recently, the Republic of Ireland did not accept the partial defense. The Irish Supreme Court had rejected the existence of the defense in DPP v O'Mahony. The case was recently abrogated, however, by enactment of the Criminal Law (Insanity) Act 2006, effective June 1, 2006. The act, in pertinent part, specifically adopted the partial defense for the charge of murder where a successful defense will result in a manslaughter conviction instead of murder.

The defense is to be contrasted with insanity which is a complete but affirmative defense. In most jurisdictions a defendant would be acquitted on the grounds of insanity if the defendant established to the satisfaction of the jury that he suffered from such a mental disease or defect that he was unable to appreciate the consequences of his actions or did not know what he was doing was wrong. As noted a successful insanity defense will result in acquittal although a number of jurisdictions have adopted the guilty but insane verdict. The defense of insanity and diminished capacity although clearly distinct are not inconsistent defenses and both may be at issue in the same case. The critical distinctions are that diminished capacity is a partial, negating defense (negates an element of the state's case) with the burden on the state to show that the defendant acted with the requisite state of mind while insanity is a complete but affirmative defense—the defendant bearing the burden of proving that he was legally insane.