Understanding temporal relations among posttraumatic stress disorder (PTSD) symptom dimensions has received increasing attention in research. However, current findings in this area are limited by group-level approaches, which are based on inter-individual variation. PTSD is a heterogeneous syndrome and symptoms are likely to vary across individuals and time. Thus, it is important to examine temporal relations among PTSD symptom dimensions as dynamic processes and at the level of intra-individual variation. The aim of the present study was to capture temporal dynamics among PTSD symptom dimensions at an individual level using unified structural equation modeling (uSEM). World Trade Center (WTC) 9/11 responders (N = 202) oversampled for current PTSD (18.3% met criteria in past month) were recruited from the Long Island site of the WTC health program. Using ecological momentary assessment (EMA), PTSD symptoms were assessed three times a day over seven consecutive days. The person-specific temporal relations among PTSD symptom dimensions were estimated with individual-level uSEM. For the sample as a whole, hyperarousal played a key role in driving the other three symptom dimensions longitudinally, with the strongest effect in intrusive symptoms. However, daily temporal relations among PTSD symptoms were idiosyncratic. Although hyperarousal was a strong predictor of subsequent symptom severity, only 33.95% of the sample showed this predictive effect while others showed more evident temporal relations between intrusion and avoidance. Implications for personalized health care and recommendations for future research using individual-level uSEM in psychopathology are discussed.