ABSTRACTObjectivePerceived burdensomeness (PB) and thwarted belongingness (TB), two proximal risk factors for suicide, may rise during residential eating disorder (ED) treatment when patients are separated from support and face exorbitant costs of care. In this setting, fostering motivation for treatment is challenging, and low motivation for treatment may exacerbate feelings of PB and TB. Simultaneously, PB and TB could reduce motivation for treatment, though no studies have explored this relationship longitudinally. Accordingly, this study examined associations between interpersonal needs (TB, PB) and motivation for treatment across the first 6 weeks of residential ED treatment.MethodsParticipants (n = 98) completed the Interpersonal Needs Questionnaire (INQ) and rated treatment motivation weekly. Pearson bivariate correlations examined the relationship between motivation and interpersonal needs at each timepoint. Two autoregressive cross‐lagged panel models (AR‐CLPMs) tested reciprocal relationships between these constructs longitudinally across the first 6 weeks of treatment.ResultsMotivation was significantly negatively correlated with PB and TB at all timepoints. In AR‐CLPM 1, Week 2 Motivation predicted Weeks 3 PB, then Week 3 PB predicted Week 4 Motivation. In AR‐CLPM 2, Week 2 TB predicted Week 3 Motivation, but Motivation did not predict TB at any timepoint.DiscussionThis study is the first to examine longitudinal relations between interpersonal needs and treatment motivation in residential ED care. PB and TB may influence one's motivation for treatment, although motivation and PB had a stronger reciprocal relationship than motivation and TB. Interpersonal needs should be addressed early in residential treatment to mitigate negative cycling.