BACKGROUND: Negative interpersonal interactions are associated with acute increases in ambulatory blood pressure (ABP). Yet, the mechanisms underlying this relationship are unclear. PURPOSE: This study tested whether negative interpersonal interactions predict higher ABP both in the moment and during subsequent observations, and whether increases in negative mood mediate these relations. These associations were tested among Black and Hispanic urban adults who may be at higher risk for negative interpersonal interactions as a function of discrimination. Race/ethnicity and lifetime discrimination were tested as moderators. METHODS: Using a 24-hr ecological momentary assessment (EMA) design, 565 Black and Hispanic participants (aged 23-65, M = 39.06, SD = 9.35; 51.68% men) had their ABP assessed every 20 min during daytime accompanied by an assessment of negative interpersonal interactions and mood. This produced 12,171 paired assessments of ABP and self-reports of participants interpersonal interactions, including how much the interaction made them feel left out, harassed, and treated unfairly, as well as how angry, nervous, and sad they felt. RESULTS: Multilevel models revealed that more intense negative interpersonal interactions predicted higher momentary ABP. Mediation analyses revealed that increased negative mood explained the relationship between negative interpersonal interactions and ABP in concurrent and lagged analyses. Discrimination was associated with more negative interpersonal interactions, but neither race/ethnicity nor lifetime discrimination moderated findings. CONCLUSIONS: Results provide a clearer understanding of the psychobiological mechanisms through which interpersonal interactions influence cardiovascular health and may contribute to health disparities. Implications include the potential for just-in-time interventions to provide mood restoring resources after negative interactions.