The aim of the study was to conduct a randomized control trial of a targeted, facilitated, test anxiety intervention for a group of adolescent students, and to examine the mediating role of uncertain control. Fifty-six participants (male = 19, white = 21, mean age = 14.7 years) were randomly allocated to an early intervention or wait-list control group. Participants completed the Revised Test Anxiety Scale and the Uncertain Control Scale from the Motivation and Engagement Scale at baseline, after the early intervention group had received the intervention, and again, after the wait-list control group had received the intervention. Participants showed moderate to large reductions in the worry and tension components of test anxiety, and uncertain control, after the intervention. The reduction in worry and tension was partially mediated by the reduction in uncertain control. Findings contribute to the evidence base for test anxiety interventions designed for school age populations and highlight uncertain control as an important factor in test anxiety intervention.