ObjectiveTo examine whether an empirically and theoretically derived treatment combining mindfulness- and acceptance-based strategies with behavioral approaches would improve outcomes in GAD over an empirically-supported treatment. Method This trial randomized 81 individuals (65.4% female, 80.2% identified as White, average age 32.92) diagnosed with GAD to receive 16 sessions of either an Acceptance Based Behavior Therapy (ABBT) or Applied Relaxation (AR). Assessments at pre-treatment, post-treatment, and 6-month follow-up included the following primary outcome measures: GAD Clinician Severity Rating, Structured Interview Guide for the Hamilton Anxiety Rating Scale, Penn State Worry Questionnaire, Depression Anxiety Stress Scale, and the State Trait Anxiety Inventory. Secondary outcomes included the Beck Depression Inventory-II, Quality of Life Inventory, and number of comorbid diagnoses. Results Mixed Effect Regression Models showed significant large effects for Time for all primary outcome measures (d’s 1.36 to 1.61) but non-significant, small effects for Condition and Condition X Time (d’s 0.002 to 0.24), indicating clients in both treatments improved comparably over treatment. For secondary outcomes, Time was significant (d’s 0.74 to 1.38) but Condition and Condition X Time effects were not (d’s 0.11 to 0.31). No significant differences emerged over follow-up (d’s 0.02 to 0.16) indicating maintenance of gains. Between 63.3 and 80.0% of clients in ABBT and 60.6 and 78.8% of clients in AR experienced clinically significant change across 5 calculations of change at post-treatment and follow-up.