Objectives: Physical activity can prevent health risks and even a slight increase in physical activity benefits health. This study investigated potential contributing factors to incidental physical activity. Design: A two-part correlational study examined whether motivational properties (autonomous and controlled motivation) in interaction with implicit processes (implicit attitudes, attentional, and approach-avoid biases) contribute to incidental physical activity. Methods: Participants (N = 103) recorded a 7-day step count to measure incidental physical activity. Implicit attitudes, attentional, and approach-avoid biases were measured using the SC-IAT, dot probe, and manikin tasks, respectively. Autonomous and controlled motivation were measured using the Perceived Locus of Causality Questionnaire. Results: Implicit attitudes and autonomous and controlled motivation were independently associated with incidental physical activity. Both autonomous and controlled motivation (when controlling for the other motivation-type) in interaction with approach bias contributed to incidental physical activity levels; motivation was positively associated with step count in participants with high avoid bias scores. Motivation did not interact with attentional bias to contribute to levels of incidental physical activity when controlling for motivation-type. Conclusions: Findings showed that elements from self-determination theory and dual process models relate to incidental physical activity behaviour. Specifically, autonomous motivation and certain implicit processes contributed to incidental physical activity engagement. This study provides an important first step towards understanding the psychological mechanisms that contribute to incidental physical activity. Statement of contribution What is already known on this subject? Motivation and implicit processes individually contribute to incidental physical activity behaviour. Autonomous motivation, rather than controlled motivation, primarily contributes to physical activity behaviour. What does this study add? Motivational and implicit processes together contribute to predicting levels of incidental physical activity. Both autonomous and controlled motivation contribute to levels of incidental physical activity. Certain implicit processes also play a role in incidental physical activity engagement.