Traumatic brain injury (TBI) involves primary mechanical damage and delayed secondary damage caused by vascular dysfunction and neuroinflammation. Intracellular components released into the parenchyma and systemic circulation, termed danger-associated molecular patterns (DAMPs), are major drivers of vascular dysfunction and neuroinflammation. These DAMPs include cell-free RNAs (cfRNAs), which damage the blood-brain barrier (BBB), thereby promoting edema, procoagulatory processes, and infiltration of inflammatory cells. We tested the hypothesis that intraperitoneal injection of Ribonuclease-1 (RNase1, two doses of 20, 60, or 180 µg/kg) at 30 min and 12 h after controlled-cortical-impact (CCI) can reduce secondary lesion expansion compared to vehicle treatment 24 h and 120 h post-CCI. The lowest total dose (40 µg/kg) was most effective at reducing lesion volume (- 31% RNase 40 µg/kg vs. vehicle), brain water accumulation (- 5.5%), and loss of BBB integrity (- 21.6%) at 24 h post-CCI. RNase1 also reduced perilesional leukocyte recruitment (- 53.3%) and microglial activation (- 18.3%) at 120 h post-CCI, but there was no difference in lesion volume at this time and no functional benefit. Treatment with RNase1 in the early phase following TBI stabilizes the BBB and impedes leukocyte immigration, thereby suppressing neuroinflammation. RNase1-treatment may be a novel approach to delay brain injury to extend the window for treatment opportunities after TBI.