Cross-sectional studies and prospective studies with long follow-up periods (e.g., years) have shown that lower levels of social support are associated with nonsuicidal self-injury (NSSI) among adolescents. This study examined how short-term changes in social support may contribute to NSSI behavior and whether different sources of support (e.g., friends, family members) provide differential protective effects against NSSI. We examined fluctuations in NSSI and social support perceived from multiple sources among a sample of 118 high-risk adolescents hospitalized for serious self-harm risk. Participants provided daily reports of social support and any self-injurious behavior for the duration of their inpatient treatment (721 total observations, average observations per participant = 6.11). Multi-level models were used to assess variability in social support and how these fluctuations relate to whether or not an individual engages in NSSI. Over one-third of participants reported engaging in NSSI at least once during inpatient hospitalization and self-reported social support varied within person across sources of support (ICC range = 0.68–0.81). Support perceived from family members and inpatient unit staff was inversely associated with NSSI, but no relationship was found between NSSI and support from other patients on the unit or friends outside of the unit. These findings suggest that the protective effects of social support for NSSI vary over short periods of time and that support perceived from adults is particularly relevant among this high-risk clinical sample. This study represents an important step in identifying risk factors to improve the detection and prevention of NSSI among adolescent inpatients.