Positive parenting programmes (PPP), albeit effective, are not readily accessible to the general public, particularly during the COVID-19 pandemic. In 103 healthy caregiver-child dyads, we investigated the effectiveness of online PPP on parenting sense of competencies (primary outcome), parenting styles and behavioural concerns of children aged 3–6 years (secondary outcomes) between 2 blinded, parallel groups. After block of 4 randomisations, intervention group (n = 52) attended live, group-based, internet delivered PPP while both intervention and active control group (n = 51) received weekly general education via communication application. Outcomes were measured at baseline, 8 and 14 weeks. Most parents from both groups had high education and household income. From the intervention group, 87.5% of the parents attended live sessions while 8.6% subsequently watched recorded sessions. At 14 weeks, the intervention group reported higher sense of competence (Wald 9.63, p = 0.008); both groups reported using more authoritative parenting style (Wald 15.52, p ≤ 0.001) from Generalised Estimating Equations model. Compared to baseline, both groups had significant reduction of children’s emotional problems at 14 weeks (mean change: Intervention = − 0.44, p = 0.033; Control = − 0.30, p = 0.046) and behavioural problems over time (Wald 7.07, p = 0.029). Online PPP offered an easily accessible, primary preventive measure to mitigate behavioural concerns and improve parental competency.Clinical trial registration Thai Clinical Trials Registry; https://www.thaiclinicaltrials.org/; TCTR20201030001; October 30, 2020.