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Abstract. Depression is a pervasive psychiatric problem following mild traumatic brain injury (mTBI). However, the onset and course of symptom expression following mTBI can differ from that of spontaneous episodes of depression. Here, we aimed to assess a physiological metric closely linked to depression: respiratory sinus arrhythmia (RSA), a measure of high frequency heart rate variability. RSA is reduced during depressive episodes, and higher resting RSA has been shown to predict future recovery from depression. In this study, we investigated if these patterns were observed throughout the typical timeframe of sub-acute mTBI recovery. Although RSA did not differ between mTBI ( n = 50) and control ( n = 27) groups, depressive symptoms were reliably correlated with RSA only in the mTBI group. This pattern was consistent 2 weeks, 2 months, and 4 months post-injury. Furthermore, resting RSA shortly following injury predicted the trajectory of depressive symptoms 2 months later. These findings generalize the connection between RSA and depression to a clinical population where depressive symptoms are common but often difficult to parse from other post-trauma consequences.