INTRODUCTION: Exposure is a central component in the treatment of a range of mental disorders. However, despite high efficacy and efficiency, dissemination of exposure-based treatments is limited. Important factors that contribute to this limited dissemination are negative beliefs about exposure on the part of the public, the therapists, and the patients. While patients perceive exposure therapy as burdensome, therapists are concerned about putting too much strain on their patients during exposure, leading to suboptimal delivery of exposure. In a previous study, in which healthy participants underwent a differential fear conditioning paradigm, we found initial evidence that the integration of a therapy dog into exposure reduces participants' anxiety and increases participants' positive affect without causing poor treatment outcome. Thus, the integration of a therapy dog into exposure might be a promising approach to address patients' and therapists' concerns and, thus, to (1) foster dissemination of exposure that is (2) delivered in an optimal manner. To scrutinise our findings in a clinical sample, we designed the present study. We test the following hypotheses: (H1) participants in the dog group report significantly less anxiety during the course of the treatment than participants in the control group. (H2) Participants in the dog group report significantly more positive affect during the course of the treatment than participants in the control group. (H3) Participants in the dog group report significantly higher therapy motivation than participants in the control group. (H4) Participants in the dog group report significantly lower anticipatory anxiety than participants in the control group. (H5) The treatment in the dog group is not inferior to the treatment in the control group. METHODS AND ANALYSIS: In this parallel randomised controlled trial of two groups, =88 participants (spider phobics without: a current diagnosis of a mental disorder other than a specific phobia, insect bite allergy, dog hair allergy, fear of dogs, current psychopharmacological treatment, and current psychotherapeutic treatment; the sample size calculation is based on the results from our previous study) are randomly allocated (with a 1:1 allocation as per a computer-generated randomisation schedule) to either an ambulant one-session in vivo exposure treatment of spider phobia with a therapy dog (dog group) or without a dog (control group). Due to the nature of the intervention, neither participants nor therapists can be blinded once participants are allocated to one of the two groups. However, the person conducting screening and diagnostics is blind to the allocation, participants are blind to the hypotheses and the respective other group, and the researchers are blind to the allocation while analysing the data. We will test (H1) and (H2), concerned with our primary outcomes, by means of 2×4 mixed analyses of variance with the between-subjects factor group (dog group vs. control group), the within-subjects factor time (with four levels, one for each time point anxiety and affect are measured during treatment), and anxiety or positive affect as the dependent variable, respectively. We will test (H3) and (H4) by means of an analyses of covariance with therapy motivation/anticipatory anxiety at baseline as the covariate, the between-subjects factor group (dog group vs. control group) and therapy motivation/anticipatory anxiety at pre-treatment as the dependent variable, respectively. We will test (H5) by means of 95% CIs and non-inferiority zones. ETHICS AND DISSEMINATION: This trial was approved by our university's ethics committee (reference number 24-11). Any deviations from this study protocol or the preregistrations as well as any adverse events potentially arising in the course of the trial, will be made explicit in the publication of the trial results. All participants provided written informed consent prior to the inclusion into the trial. The findings from this trial will be disseminated by means of common academic pathways, including peer-reviewed publications and conference presentations. Following common open science practices, data and analysis code will also be made publicly available in anonymised form on the Open Science Framework (osf.io). TRIAL REGISTRATION NUMBER: On 18 June 2024, this study was registered at the German Clinical Trials Register (ID: DRKS00034494; https://drks.de/search/de/trial/DRKS00034494) and preregistered at AsPredicted (https://aspredicted.org/JRP_SCF).