Posttraumatic stress disorder (PTSD) frequently co-occurs with addictive behaviors. Recently, the addictive behavior of excessive smartphone use is being widely researched. Impulsivity commonly relates to PTSD severity and problematic smartphone use. However, unexamined is the mediating role of impulsivity facets (lack of premeditation, negative urgency, sensation seeking, and lack of perseverance) in the PTSD-problematic smartphone use relationship; this was the purpose of the current study. We used data collected from 346 participants recruited through Amazon's Mechanical Turk (MTurk) platform. PTSD severity, impulsivity facets, and problematic smartphone use were assessed using the PTSD Checklist for DSM-5 (PCL-5), the UPPS Impulsive Behavior Scale, and the Smartphone Addiction Scale-Short Version (SAS-SV), respectively. Negative urgency and lack of perseverance had significant positive correlations with both PTSD severity and problematic smartphone use. Results of mediation analyses indicated that negative urgency significantly mediated the relationship between PTSD severity and problematic smartphone use. Findings support the underlying role of negative urgency in the relation between PTSD severity and problematic smartphone use. Thus, treatments targeting negative urgency may potentially serve to reduce problematic smartphone use among individuals experiencing PTSD symptoms.