Objective: Childhood trauma is linked to the dysregulation of physiological responses to stress, particularly lower cardiovascular reactivity (CVR) to acute stress. The mechanisms that explain this association, however, are not yet fully understood. Method: Using secondary data from the Midlife in the United States (MIDUS) Biomarker Project (N = 1,148; n = 652 females), we examine whether social integration can help explain the association between childhood trauma and lower CVR. Participants completed a standardized laboratory stress paradigm which involved completing executive functioning (Stroop) and mental arithmetic (MATH) tasks. Cardiovascular measurements were continuously assessed using electrocardiogram (ECG) and Finometer equipment. The Social Well-Being Scale (Keyes, 1998) and the Childhood Trauma Questionnaire (CTQ; Bernstein et al., 2003) measured social integration and trauma, respectively. Results: Regression analyses demonstrated that childhood trauma was associated with lower systolic (SBP; ß = -.14, p <.001) and diastolic (DBP; ß = -.11, p <.001) blood pressure reactivity but not heart rate (HR) reactivity. Mediation analyses, using Hayes PROCESS Model 4, showed that higher levels of trauma were associated with less social integration and in turn linked to lower reactivity across all biological indices. Moreover, sensitivity analyses showed that this indirect effect via social integration was evident for emotional and physical abuse, emotional and physical neglect, but not sexual abuse. Conclusion: Overall, the results indicated that dysregulated cardiovascular stress responses owing to childhood trauma may be shaped by a lack of social integration. The implications of this, as well as the findings for the individual types of trauma, are discussed.