Subthreshold posttraumatic stress disorder (PTSD) has long been recognized as an important construct that identifies a subgroup of individuals who report significant PTSD symptoms and associated disability but do not endorse enough symptoms to meet the criteria for a full PTSD diagnosis. Different investigators have defined subthreshold PTSD in various ways, making it difficult to interpret findings across studies. To address this problem, we systematically compared individuals who met criteria for nine different subthreshold PTSD definitions with individuals diagnosed with either full PTSD or no PTSD (i.e., failed to meet the criteria for a subthreshold definition) with respect to prevalence and associated clinical outcomes of interest. Participants were 1,082 veterans enrolled in the Veterans After Discharge Longitudinal Registry. PTSD and subthreshold PTSD diagnostic status were determined using the Structured Clinical Interview for DSM-5 (SCID-5) and validated self-report instruments were used to assess clinical outcomes. Across outcomes, subthreshold definitions generally identified a group of participants that was distinguishable from participants in both the PTSD and no PTSD groups, rs = .02–.47. We discuss the benefits and drawbacks of various subthreshold definitions and highlight the need for additional work evaluating these definitions across additional outcomes and samples. In the interim, we propose a working case definition of subthreshold PTSD as meeting any three of the four DSM-5 symptom criteria (i.e., Criteria B, C, D, and E) along with Criterion A and Criteria F–H. The results suggest subthreshold PTSD is a clinically meaningful construct.