Background: Anxiety symptoms in older adults are prevalent and disabling but often go untreated. Most trials on psychological interventions for anxiety in later life have examined the effectiveness of face-to-face cognitive behavioral therapy (CBT). To bridge the current treatment gap, other treatment approaches and delivery formats should also be evaluated. Objective: This study is the first to examine the effectiveness of a brief blended acceptance and commitment therapy (ACT) intervention for older adults with anxiety symptoms, compared with a face-to-face CBT intervention. Methods: Adults aged between 55-75 years (n=314) with mild to moderately severe anxiety symptoms were recruited from general practices and cluster randomized to either blended ACT or face-to-face CBT. Assessments were performed at baseline (T0), posttreatment (T1), and at 6- and 12-month follow-ups (T2 and T3, respectively). The primary outcome was anxiety symptom severity (Generalized Anxiety Disorder-7). Secondary outcomes were positive mental health, depression symptom severity, functional impairment, presence of Diagnostic and Statistical Manual of Mental Disorders V anxiety disorders, and treatment satisfaction. Results: Conditions did not differ significantly regarding changes in anxiety symptom severity during the study period (T0-T1: B=.18, P=.73; T1-T2: B=-.63, P=.26; T1-T3: B=-.33, P=.59). Large reductions in anxiety symptom severity (Cohen d≥0.96) were found in both conditions post treatment, and these were maintained at the 12-month follow-up. The rates of clinically significant changes in anxiety symptoms were also not different for the blended ACT group and CBT group (X 2 1=0.2, P=.68). Regarding secondary outcomes, long-term effects on positive mental health were significantly stronger in the blended ACT group (B=.27, P=.03, Cohen d=0.29), and treatment satisfaction was significantly higher for blended ACT than CBT (B=3.19, P<.001, Cohen d=0.78). No other differences between the conditions were observed in the secondary outcomes. Conclusions: The results show that blended ACT is a valuable treatment alternative to CBT for anxiety in later life.