Objectives: This longitudinal study applied the integrated model of self-determination theory (SDT) and the theory of planned behaviour (TPB) to explain COVID-19 preventive behaviours among parents of young children in the United States. Design: The study adopted a two-wave longitudinal study design. Parents (N = 681) completed self-report questionnaires related to measures of SDT and the TPB constructs and behavioural adherence at baseline and after one month. We used standardised residual change scores to test the structural relationships of the integrated model. Results: The parameter estimates of the model (CFI >.96, TLI >.86, RMSEA =.05, SRMR =.03) fit acceptably well to the data. Psychological need support was positively and significantly linked to autonomous and controlled motivation and amotivation. Autonomous motivation was positively and significantly correlated with TPB factors, and intention. Intention was a significant and positive predictor of behavioural adherence. Conclusion: The integrated model of SDT and the TPB appeared to be applicable to the explanation of COVID-19 prevention among the U.S. parents. Longitudinal data showed that a psychological need supportive social environment was related to favourable motivation, social cognition beliefs, intention and behavioural adherence to the preventive behaviours of parents protecting their young children from COVID-19.