Background: Relationship satisfaction has been identified as an important factor in terms of extradyadic sexual involvement. However, in men, fatherhood might be associated with infidelity by leading to changes in relationship satisfaction and the social life of parents. To date, no study has focused on the association of fatherhood and infidelity, nor the influence of fatherhood on the association between relationship satisfaction and infidelity. Methods: Using a cross-sectional design, 137 fathers and 116 non-fathers were assessed regarding relationship satisfaction, infidelity, and potential confounds. Results: Significantly more fathers reported having been unfaithful in the current relationship than non-fathers (30.7% vs. 17.2%). Fathers also reported longer relationship duration, higher relationship satisfaction, and lower neuroticism than non-fathers. Furthermore, fatherhood moderated the association between relationship satisfaction and infidelity insofar that only in non-fathers reduced relationship satisfaction was associated with infidelity. Conclusions: The results suggest that fatherhood increases the risk of engaging in extradyadic sexual activities and moderates the link between relationship satisfaction and infidelity. However, results need to be interpreted with caution due to the cross-sectional study design and the lack of information about the specific time point of the infidelity incident(s).