Background Professional identity formation (PIF) is a central developmental task of residency, yet curricula that support identity work are predominantly synchronous and facilitator-dependent. Sternszus and colleagues have reconceptualized Objective To evaluate whether a brief, longitudinal, asynchronous professionalism curriculum delivered across all three postgraduate years of a large pediatric residency was associated with gains in self-assessed PIF-related capabilities, and to characterize residents' identity work using directed content analysis guided by the being–becoming–belonging framework. Methods Level-specific two-hour asynchronous modules (narrated didactics, required readings, case vignettes, podcast reflection, identity mapping with anchor-value selection) were completed by PGY1–PGY3 residents during assigned didactic weeks in academic year 2025–2026. Pre/post assessments included level-specific Likert items and open-ended prompts. PGY2 responses were individually paired (Wilcoxon signed-rank); anonymous PGY1 and PGY3 responses were compared at the group level (Mann-Whitney U). Narrative responses (n=144 surveys; 63 extended reflections) underwent directed content analysis. Results Eighty pre- and 64 post-assessments were completed. Among 28 paired PGY2 residents, composite self-assessment increased from 4.04 to 4.53 of 5 (P<.001; r=0.83 [large effect]); 22 of 28 residents increased. All 10 PGY3 items improved (pooled strongly-agree responses 17% pre vs 60% post, P<.001), with a smaller pooled gain for PGY1 (P=.007). Qualitatively, residents entered defining professionalism behaviorally (54% of definitions used behavior/conduct language) and used the modules to do identity work. Conclusions An asynchronous curriculum embedded across residency was feasible, was consistently rated as meaningful, and was associated with significant gains in self-assessed PIF capabilities.