Background <p>Aging challenges healthcare globally. Fish and shellfish (fish-shellfish) are substantial providers of omega-3 fatty acids, which may mitigate multiple aging-related diseases. However, the relationship between fish-shellfish consumption and biological aging (BA) remains incompletely elucidated.</p> Aims <p>To explore potential relationships between total fish-shellfish consumption (status/frequency) and BA assessed via Phenotypic age (PhenoAge) and risk of PhenoAge acceleration (PhenoAgeAccel), with a focus on examining the mediation role played by inflammatory/antioxidant biomarkers.</p> Methods <p>The cross-sectional study was executed using weighted data from 27,801 American adults who participated in the National Health and Nutrition Examination Survey (1999–2020). To assess the total fish-shellfish consumption–BA relationship, multivariable linear and logistic regression, restricted cubic spline (RCS), threshold effect analysis, mediation analysis, and subgroup analysis were used.</p> Results <p>Total fish-shellfish consumption inversely associated with BA. Specifically, fish consumers had significantly lower PhenoAge (β = -1.01, 95% CI: -1.41, -0.62) and reduced PhenoAgeAccel risk (OR = 0.84, 95% CI: 0.77, 0.92; both <i>P</i> &lt; 0.001) versus non-consumers. Higher fish consumption frequency also associated with attenuated BA (β_PhenoAge = -0.38, 95% CI: -0.57, -0.19; OR_{PhenoAgeAccel} = 0.94, 95% CI: 0.90, 0.99; <i>P</i> &lt; 0.05), with RCS and threshold analyses revealing an L-shaped negative relationship. Inflammatory and antioxidant biomarkers mediated 7.98%–27.37% of the fish consumption–BA association. However, shellfish consumption showed no significant link to BA.</p> Conclusions <p>Our study demonstrated a negative correlation between total fish-shellfish consumption and BA, with a potential primary contributor of fish consumption to this association. Inflammatory/antioxidant biomarkers may mediate the relationship.</p>