Background Non-suicidal self-injury (NSSI) is a potent risk factor for suicide among youth. There is limited research, however, on the association between NSSI and suicide risk among adults, particularly among adult medical patients, who are a population at increased risk for suicide. To address this research gap, the current analysis aimed to describe the association between lifetime history of NSSI and suicide risk in an adult medical inpatient population. Method Adult medical inpatients aged 18 or older from one of four United States hospitals were screened for suicide risk and a lifetime history of NSSI. Suicide risk was determined using the Ask Suicide-Screening Questions (ASQ). NSSI history, methods, frequency, and severity were assessed via a structured interview based on the Self-Injurious Thoughts and Behaviors Interview. Results A total of 621 adult medical inpatients were included in this secondary analysis (55.2% male; 60.9% White; M[SD] age = 50.3[16.7]); 5.8% of patients (36/621) reported a lifetime history of NSSI and 16.1% (100/621) screened positive for suicide risk. Patients with a lifetime history of NSSI were significantly more likely to screen positive for suicide risk (OR = 9.4 [95% CI, 4.4–20.8]; p < 0.0001). Limitations This analysis used cross sectional data and could not examine any causal relationships between NSSI and suicide risk. Conclusions Adult medical inpatients with a lifetime history of NSSI were significantly more likely to screen positive for suicide risk. Research examining NSSI among adult medical patients is especially relevant for suicide risk detection and prevention efforts.