Romantic and peer relationships both contribute to overall well-being; however, each demand time and emotional investment that may give rise to competition. Little is known about how men, in particular, balance these relationships. We explored the extent to which men’s investments in peer relationships are associated with romantic relationship quality. We further examined differences between fathers and non-fathers. Data were from five annual waves of the Men and Parenting Pathways cohort study ( N = 608). Participants were men in committed relationships ( n = 526) aged 28-32 years at baseline ( M = 29.91, SD = 1.3). In cross-sectional, linear regressions, estimated using Generalised Estimating Equations (GEE; to account for repeated waves), we investigated peer network investments, (1) time (hours) spent with peers, (2) close network size, and (3) extended network size, and their associations with romantic relationship quality, self-reported using the Dyadic Adjustment Scale. Inverted U-shaped associations were found for time spent with peers and close network size, whereby increases in investment were associated with improved relationship quality up to a “tipping point”, following which declines in quality are observed. For extended social networks, the association was linear, such that larger networks were linked to higher romantic relationship quality; however, this association was only evident in fathers. Investing too little or too much in peer relationships may adversely impact the quality of men’s relationships with romantic partners. Supporting men, and particularly fathers, to maintain appropriate investment in peer networks may have benefits for their romantic relationships.