Exposure to fine particulate matter (PM2.5 ) air pollution has been associated with skin-related diseases or disorders.To evaluate the potential skin protective effects of fish-oil supplementation against PM2.5 exposure.This is an exploratory analysis based on a pilot randomised, double-blind, placebo-controlled trial among 65 healthy young adults between September 2017 and January 2018 in Shanghai, China. We randomly assigned participants to intake either 2.5 g/day of fish oil or placebo for consecutive 4 months. Four rounds of skin D-squame tape samples were collected in the last 2 months, and 5 secondary biomarkers of skin inflammation and oxidative stress were measured. Fixed-site PM2.5 concentrations on campus were measured in real time. We used linear mixed-effect models to analyse the associations between short-term PM2.5 exposure and biomarkers in each group.The 24-h average PM2.5 concentration was 34.68±15.83 μg/m3 . There were generally weaker associations between PM2.5 and biomarkers in the fish-oil group than in the placebo group, but the associations and the between-group differences varied by biomarkers and lag periods. Compared to the placebo group, for a 10-μg/m3 increase in PM2.5 , the increments of interleukin-1α and carbonyl protein in the fish-oil group were 41.55% smaller (95% confidence interval: 4.61%, 78.48%) at lag 0-48 h and 22.01% smaller (95% confidence interval: 11.25%, 32.77%) at lag 0-24 h, respectively. No significant between-group differences were observed for other biomarkers.This study suggested that dietary fish-oil supplementation may improve biomarkers of skin inflammation and oxidative-stress response to short-term PM2.5 exposure.