Objective: To investigate the longitudinal associations between COVID-19 induced stress (related to COVID-19 restrictions/changes), attention-deficit/hyperactivity disorder (ADHD) symptoms, oppositional symptoms, and mental health outcomes (negative affect, anxiety, depression, and irritability) in children with ADHD during the COVID-19 pandemic. Method: Parents of 140 Australian children with ADHD (aged 5–17 years) completed an online survey in May 2020 during stay-at-home restrictions and 12-months later. Results: Baseline COVID-19 stress was associated with increased total ADHD symptom severity (β = .21, p = .007) and hyperactivity/impulsivity symptoms (β = .23, p = .002) at 12-months, after accounting for covariates (i.e., child age, gender, ADHD medication, socio-economic status, and baseline symptoms). Despite some indication of associations between baseline COVID-19 stress and 12-month oppositional symptoms and negative affect, these were attenuated when adjusting for baseline symptoms. Conclusions: The study provides initial evidence of the medium-term impacts of pandemic-related stress for children with ADHD.