A nonblinded randomized trial was conducted at two Canadian provincial outpatient addiction clinics that tested the effectiveness of a systemic congruence couple therapy (CCT) versus individual‐based treatment‐as‐usual (TAU) on nine clinical outcomes: (1) primary outcomes—alcohol use and gambling, psychiatric symptoms, and couple adjustment; (2) secondary outcomes—emotion regulation, substance use, depression, post‐traumatic stress symptoms, and life stress. Data of primary clients and partners (N = 46) were analyzed longitudinally across baseline, posttreatment (5 months), and follow‐up (8 months). Alcohol use disorder (95%) and gambling disorder (5%) were in the severe range at baseline, and co‐addiction was 27%. Psychiatric comorbidity was 100%, and 18% of couples were jointly addicted. Between‐group comparison favored CCT in primary outcomes with medium‐to‐large effect sizes (Cohen's h = 0.74–1.44). Secondary outcomes were also significantly stronger for CCT (Cohen's h = 0.27–1.53). Within‐group, for all primary outcomes, a significant proportion of symptomatic CCT clients and partners improved, converging with ANOVA results of large effect sizes (0.14–0.29). All secondary outcomes improved significantly in CCT with large effect sizes (0.14–0.50). TAU showed significant within‐group improvement in alcohol use, other substance use, and life stress with large effect sizes (0.16–0.40). Primary clients and partners made largely equivalent improvement within CCT and within TAU. Results were triangulated with clients' satisfaction ratings and counselors' reports. Overall, significant within‐group effects were detected for CCT both clinically and statistically and between‐group difference favored CCT. Future trials are required to validate these promising findings.