Objective Despite interpersonal psychotherapy's (IPT) efficacy for depression, little is known about its change‐promoting ingredients. This exploratory study examined candidate change mechanisms by identifying whether patients’ interpersonal and cognitive characteristics change during IPT and whether such changes relate to outcomes. Method Patients were 95 depressed adults receiving manualized IPT. We used multilevel modeling to assess the relation between change in each interpersonal and cognitive domain and outcome. Results Across all interpersonal and cognitive variables measured, patients showed significant improvement. Unexpectedly, reduced romantic relationship adjustment was related to posttreatment depression reduction (β = 2.028, p = .008, self‐rated; β = 1.474, p = .022, clinician‐rated). For the other measured domains, change was not significantly associated with outcome (though changes in some interpersonal variables evidenced a trend‐level relation to outcome). Conclusions Possible reciprocal influences among IPT, depression, and romantic relationship functioning are discussed, as are implications for future research.