The ability of an individual to participate in courtroom proceedings is assessed by clinicians using legal ‘fitness to plead’ criteria. Findings of ‘unfitness’ are so rare that there is considerable professional unease concerning the utility of the current subjective assessment process. As a result, mentally disordered defendants may be subjected unfairly to criminal trials. The Law Commission in England and Wales has proposed legal reform, as well as the utilisation of a defined psychiatric instrument to assist in fitness to plead assessments. Similar legal reforms are occurring in other jurisdictions. Our objective was to produce and validate a standardised assessment instrument of fitness to plead employing a filmed vignette of criminal proceedings. The instrument was developed in consultation with legal and clinical professionals, and was refined using standard item reduction methods in two initial rounds of testing (n = 212). The factorial structure, test-retest reliability and convergent validity of the resultant instrument were assessed in a further round (n = 160). As a result of this iterative process a 25-item scale was produced, with an underlying two-factor structure representing the foundational and decision-making abilities underpinning fitness to plead. The sub-scales demonstrate good internal consistency (factor 1: 0·76; factor 2: 0·65) and test-retest stability (0·7) as well as excellent convergent validity with scores of intelligence, executive function and mentalising abilities (p≤0·01 in all domains). Overall the standardised Fitness to Plead Assessment instrument has good psychometric properties. It has the potential to ensure that the significant numbers of mentally ill and cognitively impaired individuals who face trial are objectively assessed, and the courtroom process critically informed.