The growing use of Rational Emotive Behavior Therapy (REBT) in performance contexts (e.g., business, sport) has highlighted the absence of a contextually valid and reliable measure of irrational beliefs. This paper reports the development and initial validation of the Irrational Performance Beliefs Inventory (iPBI). The iPBI was developed to provide a validated measure of the four core irrational beliefs of REBT theory. Item development was completed in three stages comprising two expert panels and one novice panel, reducing and refining 176 items to 133. Then, exploratory and confirmatory factor analyses were used to refine the measure and reduce the number of items. A total of 665 business professionals completed the 133-item scale, alongside an established measure of irrational beliefs and a measure of negative emotion. A 28-item measure was developed (the iPBI) that showed an acceptable fit to the four-factor REBT structure. The iPBI correlated well with the established irrational beliefs measure, and with anxiety, depression, and anger, demonstrating concurrent and predictive validity. Further validation efforts are required to assess the validity and reliability of the iPBI in alternative samples in other performance-related contexts. Rational Emotive Behavior Therapy (REBT) predicts that in adverse conditions it is not the situation that causes dysfunctional emotions, but rather individual beliefs about the situation (Ellis, 1957). In REBT, dysfunctional emotions (e.g., immobilizing emotions such as anxiety) and associated maladaptive behaviours (e.g., withdrawal) stem from irrational beliefs (see e.g., Browne, Dowd, & Freeman, 2010;Szentagotai & Jones, 2010). In contrast, functional emotions (e.g., mobilizing emotions such as concern) and associated adaptive behaviours (e.g., assertiveness) stem from rational beliefs (Ellis, Gordon, Neenan, & Palmer, 1997). In current REBT theory (Dryden, 2014) there are four core irrational beliefs -one primary (demandingness) and three secondary (awfulizing, low-frustration tolerance, and depreciation). In parallel, there are four types of rational belief, one primary (preferences) and three secondary (anti-awfulizing, high-frustration tolerance, and self-acceptance). Secondary beliefs are derived from the primary belief (Dryden, 2012). The goal of REBT is to reduce irrational beliefs in favour of rational beliefs.Researchers have applied REBT to develop functional emotions and effective performance in contexts such as competitive sport (Turner, 2014;Turner & Barker, 2013) and business (Turner & Barker, 2015). However, one limitation hindering research developments in performance settings is the lack of a contextually-specific irrational beliefs measure that is short enough for test-retest purposes and aligns closely with contemporary measures, so that a more accurate understanding of the specific conditions or subpopulation with which a tool works best in can be gained. Therefore, the development of a contextually specific irrational beliefs measure should con...