We adapted the Family Check-Up Online (FCU-O): 1) to support families coping with pandemic-related stressors to prevent behavioral and emotional problems among middle school youth, and 2) for smartphone delivery to increase access and reach during the COVID-19 pandemic. This study evaluated the direct and indirect effects of the adapted FCU-O at 4-months post-baseline. The FCU-O combines online parenting support with telephone coaching. Participants were primary caregivers of children ages 10 to 14 years. Eligibility included endorsing depression on the PHQ-2 or significant stress on a 4-item version of the Perceived Stress Scale. We randomly assigned participants to the adapted FCU-O (N = 74) or a waitlist control condition (N= 87). Participants predominantly self-identified as female (95%), 42.77 years old on average, and White (84.6%). Outcomes included caregiver reports of perceived stress and parenting, and youth conduct problems and depressive symptoms. Using a multilevel modeling approach, we tested intent-to-treat intervention effects at 4-months, with time points nested within participants. The FCU-O reduced caregiver stress and improved proactive parenting and limit setting but had no effects on youth outcomes. Effect sizes were small to moderate (Cohen's d ranged from 0.37-0.57). We examined indirect effects on youth outcomes at 4-months via changes in caregiver stress and parenting at 2-months. Mediation analyses suggested indirect effects on youth depressive symptoms via reductions in caregiver stress and increases in proactive parenting. Results indicate the FCU-O has potential as a public health intervention for families facing extreme stressors such as those during the COVID-19 pandemic. [This paper will be published in "Prevention Science."]