Introduction: Fear of root canal treatment (RCT) may affect care seeking and treatment experience but is often summarized using anxiety scores. This study identified fear severity patterns and trigger-content profiles and examined associations with avoidance and negative experiences. Materials and Methods: In this single-center cross-sectional study, 219 of 247 adults scheduled for primary RCT were analyzed after 28 exclusions. Preoperative anxiety was measured using a 0–10 cm Visual Analog Scale for Anxiety (VAS-A). Hierarchical cluster analysis identified fear severity patterns, and latent class analysis identified trigger-content profiles among participants endorsing at least one trigger. Results: At least one trigger was endorsed by 75.8%; 21.9% had severe anxiety (VAS-A ≥ 7.0). Severe anxiety increased descriptively from 1.9% with no trigger to 54.3% with three or more triggers. Hierarchical clustering identified low-fear (n = 152) and high-fear (n = 67) patterns. In bivariate comparisons, high-fear membership was more common among women, participants postponing dental visits, and those with negative childhood or adult RCT experiences, and less common among regular dental attenders. Hierarchical multivariable logistic regression showed that female sex and negative adult RCT experience were associated with higher odds of high-fear pattern membership, whereas regular dental attendance was associated with lower odds; negative childhood RCT experience and postponement were not independently significant in the fully adjusted model. Latent class analysis yielded needle-focused (n = 38) and procedural (n = 128) profiles; the latter showed a broader trigger range, while VAS-A distributions were similar. Conclusions: Fear of RCT was heterogeneous. Assessing severity, triggers, and avoidance history may support individualized communication. The profiles require external validation before use.