Background: Long COVID is clinically heterogeneous; objective biomarkers for diagnosis and biological stratification remain incompletely defined. Methods: We conducted a cross-sectional analysis of 191 de-identified post-COVID participants (158 with long COVID; 33 without sequelae), assessing eight biomarkers. Values below detection limits were imputed as half the assay-specific threshold. Analyses included two-sided Mann–Whitney U tests with Benjamini–Hochberg false-discovery-rate (FDR) correction, logistic regression, receiver operating characteristic analysis with stratified bootstrap confidence intervals (B=1,000), and Ward hierarchical clustering. Results: The long COVID group showed significantly higher IL-6, TNF-α, CXCL9, IP-10, SLAMF1, IL15RA, and IL-18 levels (all FDR q<0.001), whereas IFN-λ3 was not discriminatory (P=0.658). CXCL9 showed the highest single-marker performance (AUC 0.998, 95% confidence interval [CI] 0.989–1.000), followed by IL-6 (AUC 0.991) and IP-10 (AUC 0.973). In the stable multivariable model, higher log10-transformed TNF-α (adjusted odds ratio [aOR] 10.38, 95% CI 1.25–86.36; P=0.030), SLAMF1 (aOR 48.30, 95% CI 4.34–537.16; P=0.002), and IL15RA (aOR 14.45, 95% CI 2.52–82.83; P=0.003) remained independently associated with long COVID, whereas IL-18 was not significant (P=0.207). The model achieved an AUC of 0.995. Clustering identified high-inflammatory and moderate-inflammatory long COVID subtypes (n=79 each). Conclusions: Persistent inflammatory chemokine and cytokine elevation characterizes long COVID, with strong discrimination by CXCL9 and IL-6. Biomarker-defined subtypes support biological heterogeneity and may guide future patient stratification.