Being able to assess whether psychological therapies are delivered according to their own principles is helpful for assuring treatment quality in research and training. We aimed to develop and preliminarily test a measure of therapist fidelity to Acceptance and Commitment Therapy (ACT) that is concise in measuring key therapist behaviours, reliable, practicable and potentially applicable across therapy contexts. This measure was developed via expert consensus in a Delphi study (Study 1). Here, thirteen expert ACT practitioners (average of 11 years’ experience with ACT, half ACBS Peer reviewed ACT trainers) participated in three iterative rounds of online questionnaires. A preliminary draft of the measure was used to initiate discussion. In the first two rounds, participants rated and commented on existing items, the manual, and structure of the measure, and generated new items for consideration. In a third round, participants commented on the emergent draft of The ACT Fidelity Measure (ACT-FM). The Delphi study resulted in a 24-item measure with items structured around the three-part model of psychological flexibility (“Tri-flex”) alongside Therapist Stance. Eighty-three percent of the chosen items met the specified criteria for consensus. In Study 2, to investigate usability and preliminary psychometric properties of the ACT-FM, a separate group of nine clinicians used the ACT-FM to rate a video of an ACT therapy session. Inter-rater reliability was moderate to excellent, and based on clinician feedback, the measure was expanded to 25 items. To reach the stated aims, further work is required – particularly evaluating the utility of the ACT-FM across therapy contexts.