The TOP program is a fully implemented responsive parenting intervention for very preterm born infants. Fidelity monitoring of interventions is important for preserving program adherence, impact outcomes and to make evidence-based adaptations. The aim of this study was to develop a fidelity tool for the TOP program following an iterative and co-creative process and subsequently evaluate the reliability of the tool. Three consecutive phases were carried out. Phase I: Initial development and pilot testing two methods namely self-report and video based observation. Phase II: Adaptations and refinements. Phase III: Evaluation of the psychometric properties of the tool based on 20 intervention videos rated by three experts.The interrater reliability of the adherence and competence subscales was good (ICC.81 to .84) and varied from moderate to excellent for specific items (ICC between .51 and .98). The FITT displayed a high correlation (Spearman’s rho.79 to.82) between the subscales and total impression item. The co-creative and iterative process resulted in a clinical useful and reliable tool for evaluating fidelity in the TOP program. This study offers insights in the practical steps in the development of a fidelity assessment tool which can be used by other intervention developers.