Group‐based trajectory modeling was used to explore empirical trajectories of symptoms of posttraumatic stress disorder (PTSD) among 17,062 adult area residents/workers (nonrescue/recovery workers) enrolled in the World Trade Center (WTC) Health Registry using 3 administrations of the PTSD Checklist (PCL) over 9 years of observation. Six trajectories described PTSD over time: low‐stable (48.9%), moderate‐stable (28.3%), moderate‐increasing (8.2%), high‐stable (6.0%), high‐decreasing (6.6 %), and very high‐stable (2.0%). To examine factors associated with improving or worsening PTSD symptoms, groups with similar intercepts, but different trajectories were compared using bivariate analyses and logistic regression. The adjusted odds of being in the moderate‐increasing relative to the moderate‐stable group were significantly greater among enrollees reporting low social integration (OR = 2.18), WTC exposures (range = 1.34 to 1.53), job loss related to the September 11, 2001 disaster (OR = 1.41), or unmet mental health need/treatment (OR = 4.37). The odds of being in the high‐stable relative to the high‐decreasing group were significantly greater among enrollees reporting low social integration (OR = 2.23), WTC exposures (range = 1.39 to 1.45), or unmet mental health need/treatment (OR = 3.42). The influence of severe exposures, scarce personal/financial resources, and treatment barriers on PTSD trajectories suggest a need for early and ongoing PTSD screening postdisaster.