Firefighters experience high rates of posttraumatic stress disorder (PTSD). It is imperative to identify malleable factors that protect against the development of PTSD symptoms among this population. We examined whether perceptions of belongingness broadly (Study 1) and social support from supervisors, coworkers, and family/friends specifically (Study 2) are associated with lower PTSD symptom severity among firefighters. Study 1 included 840 U.S. firefighters (91.1% male); participants completed the Interpersonal Needs Questionnaire and PTSD Checklist—Civilian Version. Study 2 included 200 U.S. women firefighters exposed to a Criterion A traumatic event; participants completed the Generic Job Stress Questionnaire, Life Events Checklist for Diagnostic and Statistical Manual of Mental Disorders–5, and PTSD Checklist for Diagnostic and Statistical Manual of Mental Disorders–5. Linear regression analyses were conducted, adjusting for the number of years participants served as firefighters. Greater belongingness broadly (Study 1; b = −0.740, p < .001) as well as social support specifically (Study 2) from supervisors (b = −4.615, p < .001), coworkers (b = −4.465, p = .001), and family/friends (b = −3.206, p = .021) were associated with less severe PTSD symptoms. When all sources of social support were entered into a single model, only support from supervisors was significantly associated with lower overall PTSD symptom severity (b = −4.222, p = .004). Belongingness and social support may protect against the development of PTSD among firefighters. Supervisor social support may be particularly salubrious, suggesting that top-down mental wellness promotion within the fire service may be indicated to protect firefighters against PTSD.