Objective: To examine the efficacy of an Internet-delivered cognitive–behavioral therapy (iCBT) program for health anxiety compared to an active psychoeducation control group. Method: Individuals (N = 86, mean age: 30 years, 87% female) with a Diagnostic and Statistical Manual of Mental Disorders (5th ed.) diagnosis of illness anxiety disorder or somatic symptom disorder with health anxiety were randomized to either a 6-lesson clinician-guided iCBT program for health anxiety (n = 45) or an active control group who received anxiety psychoeducation, clinical support, and monitoring (control, n = 41) over a 12-week period. Results: Both groups experienced significant improvements between baseline and posttreatment on self-report measures of health anxiety, depression, general anxiety, and functional impairment. Intention-to-treat analyses indicated that the iCBT group experienced greater improvements in health anxiety on the Short Health Anxiety Inventory (SHAI) compared to controls (between-groups effect size = 1.39, 95% confidence interval [0.87, 1.93]), and a greater proportion of the iCBT group showed clinically reliable change on the SHAI (84% vs. 34% in the control group). Similarly, the iCBT group outperformed the control group on secondary measures of depression, generalized anxiety, functional impairment, maladaptive cognitions, body hypervigilance, safety behaviors and avoidance, and intolerance of uncertainty. Gains were maintained at 3-month follow-up in the iCBT group. Conclusion: iCBT for health anxiety is more effective than psychoeducation, clinical support, and monitoring, and presents an efficacious and accessible treatment option for people with health anxiety.