Obesity is a major health problem in the United States and a growing concern among members of the military. Posttraumatic stress disorder (PTSD) has been associated with overweight and obesity and may increase the risk of those conditions among military service members. Disordered eating behaviors have also been associated with PTSD and weight gain. However, eating disorders remain understudied in military samples. We investigated longitudinal associations among PTSD, disordered eating, and weight gain in the Millennium Cohort Study, which includes a nationally representative sample of male (n = 27,741) and female (n = 6,196) service members. PTSD at baseline (time 1; 2001–2003) was associated with disordered eating behaviors at time 2 (2004–2006), as well as weight change from time 2 to time 3 (2007–2008). Structural equation modeling results revealed that the association between PTSD and weight change from time 2 to time 3 was mediated by disordered eating symptoms. The association between PTSD and weight gain resulting from compensatory behaviors (vomiting, laxative use, fasting, overexercise) was significant for white participants only and for men but not women. PTSD was both directly and indirectly (through disordered eating) associated with weight change. These results highlight potentially important demographic differences in these associations and emphasize the need for further investigation of eating disorders in military service members.