Embracing the Bayesian approach, we aimed to synthesise evidence regarding barriers and enablers to physical activity in adults with heart failure (HF) to inform behaviour change intervention. This approach helps estimate and quantify the uncertainty in the evidence and facilitates the synthesis of qualitative and quantitative studies. Qualitative evidence was annotated using the Theoretical Domains Framework and represented as a prior distribution using an expert elicitation task. The maximum a posteriori probability (MAP) for the probability distribution for the log OR was used to estimate the relationship between physical activity and each determinant according to qualitative, quantitative, and qualitative and quantitative evidence combined. The probability distribution dispersion (SD) was used to evaluate uncertainty in the evidence. Three qualitative and 16 quantitative studies were included (N = 2739). High pro-b-type natriuretic peptide (MAP = −1.16; 95%CrI: [−1.21; −1.11]) and self-reported symptoms (MAP = − 0.48; 95%CrI: [−0.40; −0.55]) were suggested as barriers to physical activity with low uncertainty (SD = 0.18 and 0.19, respectively). Modifiable barriers were symptom distress (MAP = −0.46; 95%CrI: [−0.68; −0.24], SD = 0.36), and negative attitude (MAP = −0.40; 95%CrI: [−0.49; −0.31], SD = 0.26). Modifiable enablers were social support (MAP = 0.56; 95%CrI: [0.48; 0.63], SD = 0.26), self-efficacy (MAP = 0.43; 95%CrI: [0.32; 0.54], SD = 0.37), positive physical activity attitude (MAP = 0.92; 95%CrI: [0.77; 1.06], SD = 0.36).