A growing body of literature suggests that specific markers of relationship quality are meaningfully linked to health outcomes. We tested whether relational self‐expansion potential might be one of these markers in cross‐sectional samples of individuals and romantic couples. Study 1 found that greater self‐expansion potential was linked to better perceived physical health via both higher positive affect (PA) and lower negative affect (NA). Study 2 replicated these findings for PA (but not NA) and revealed both actor and partner effects of self‐expansion potential. Results remained robust when statistically accounting for gender, age, body mass index, agreeableness, neuroticism, and perceived partner responsiveness. These findings identify a new relationship‐level "active ingredient" associated with health and have implications for future physical health studies. [ABSTRACT FROM AUTHOR]