Health and environment are closely related and captured together within the concept of planetary health. Hence, there is a compelling interest in targeting health and pro- environmental behaviours (PEB) together. To enhance the effectiveness of (multiple) health and PEB behaviour change jointly, this study aimed to (1) identify whether individual health and PEB are believed to facilitate each other (positive spillover) or conflict (negative spillover) with one another. Furthermore, this study also assessed (2) the predictors of these spillover beliefs. A cross-sectional survey was administered to 311 adult participants (M age=29.24 ± 9.81; 50% female) from Western Europe. A scale was developed to measure spillover beliefs. Self-reported values, health and green identity, as well as self-efficacy, pride, and guilt related to specific health and PEB were assessed as predictors. Results showed that spillover beliefs between health and PEB exist (M pos=2.97 ±.80; M neg=2.16 ±.71). Personal values, health and green identity, as well as self-efficacy and guilt for PEB, significantly predicted both types of spillover beliefs. Furthermore, pride for PEB increased the likelihood of positive spillover beliefs, however, self-efficacy, pride and guilt for health did not. These findings on spillover beliefs support further investigation of spillover within the planetary health context.