For people with Post-traumatic stress disorder (PTSD), symptoms can fluctuate daily, likely impairing day-to-day functioning. This daily flux can be monitored closely through the use of ecological momentary assessment (EMA). However, it is unclear what impacts daily fluctuation in PTSD symptoms. Anxiety sensitivity (AS), or the fear of physical, social, and cognitive arousal associated with high anxiety is one potential risk for daily fluctuation in PTSD symptoms. Research has shown AS predicts differences in PTSD symptoms between people. It is unknown whether AS also predicts within-individual differences in daily PTSD symptoms. Therefore, the current study uses EMA to examine AS effects on within-individual daily variability in PTSD symptoms. The sample was composed of 30 people (M age = 38.03, SD = 15.14; 61.3% female) diagnosed with PTSD using a structured clinical interview. Participants completed a battery of self-report measures at baseline, including baseline AS, using the Anxiety Sensitivity Index-3. Participants then completed brief assessments of PTSD symptoms four times daily, over the next 7 days. Multilevel modeling was used to examine the impact of baseline AS total and AS subscale scores on within-person daily variability in PTSD symptoms. Baseline AS total scores significantly predicted within-individual daily variability in PTSD symptoms (B = .18, p = .01). AS physical concerns (B = .50, p = .01), cognitive concerns (B = .40, p = .03), and social concerns (B = .59, p = .002), each significantly predicted within-individual daily variability in PTSD symptoms. However, none of the subscales predicted PTSD symptoms when accounting for the other subscales. Further, the impact of baseline AS total scores marginally predicted daily variability in PTSD symptoms when accounting for baseline negative affect. This research uniquely adds to the body of knowledge about the role of AS in predicting PTSD symptoms by showing AS predicts within-person daily variability of PTSD symptoms. Although this study is the first to assess AS as a predictor of within-person PTSD symptom variability, these findings are consistent with prior studies demonstrating that AS acts as a risk factor for PTSD symptoms using retrospective questionnaires. This research has translational potential as brief AS interventions have shown to reduce PTSD symptoms beyond a level that is considered clinically meaningful in targeting between-person differences. Given this, research should be conducted examining whether within-person variability is also reduced using these interventions.