University students consistently report high levels of stress, which has been associated with a range of adverse outcomes. Promoting adaptive coping behaviours, such as problem‐focused coping for managing university stress, is therefore a timely area of investigation. Current coping intervention approaches target reasoned cognitive processes; however, recent research has suggested that automatic processes are more strongly associated with problem‐focused coping behaviour. The current study examined the effect of an implementation intentions intervention, a technique that can support behaviour to be performed automatically by facilitating continued repetition of a plan, on problem‐focused coping behaviour under stress and stress‐related outcomes. Following a pilot study (N = 21), a preregistered randomized controlled trial was conducted with university students (N = 154) using an online survey. Participants completed baseline measures of problem‐focused coping behaviour, behavioural automaticity, behavioural intentions, action planning, perceived stress, procrastination, and psychological wellbeing; before receiving the intervention or control condition stimuli, and then at a 2‐week follow‐up. Behavioural intention and action planning were also measured immediately post‐intervention. The intervention had a significant medium‐sized effect on action planning for problem‐focused coping, but no other significant effects were detected. Exploratory assessment of plan quality revealed medium‐sized correlations between plan quality and changes in problem‐focused coping behaviour. Findings indicate that implementation intentions may be a promising approach for increasing planning for the use of problem‐focused coping. Indicators of plan quality found to be associated with changes in problem‐focused coping provide valuable avenues for intervention optimisation in future research.