Background: Mothers of children with autism spectrum disorder (ASD) report high levels of stress and lower levels of well-being than parents of typically developing children. Current interventions for ASD typically focus on working with the child rather than delivering strategies to help support parents. Objective: To evaluate the effectiveness of a psychoeducation intervention developed to support mothers of children with ASD in Saudi Arabia. Method: Sixty-two mothers (23-52 years) of children (26-78 months) were recruited to a multisite randomized controlled trials of the intervention. The intervention consisted of one face-to-face session (60 min) and four virtual sessions (30 min each) delivered using WhatsApp. Parenting stress was the primary outcome, with secondary outcomes focusing on maternal depression, anxiety, and happiness, and child behaviour problems and ASD symptoms. Data were collected at baseline T1, immediately postintervention T2 and 8-week follow-up T3. Results: One-way analysis of covariance was used at T2 and T3 with T1 scores entered as a covariate. Improvements were found at T2 for stress (F = 234.34, p = .00, and d = -1.52) and depression (F = 195.70, p = .00, and d = -2.14) but not anxiety, and these results were maintained at T3. Changes in child behaviour problems were limited to improvements in hyperactivity at T2 (F = 133.66, p = .00, and d = -1.54). Although changes in stress and depression were statistically significant, change to clinically normal levels was limited to depression. None of the participants had recovered after the intervention (Parent Stress Index Short Form stress scores), whereas 23 mothers (71.87%) in the intervention group had recovered at T2 and 22 (68.75%) at T3 (Hospital Anxiety and Depression Scale depression scores). Conclusion: This intervention with WhatsApp support is beneficial but may need to be augmented with other forms of support for mothers of children with ASD including more condensed sessions on stress and interventions targeting anxiety.