Air pollution has been reported to be an environmental risk factor for autism spectrum disorder. However, the gut microbiome's role as a potential mediator has not been investigated. We aimed to clarify whether particulate matter with an aerodynamic diameter ≤10 μm (PM 10) and nitrogen dioxide (NO 2) exposure impact autistic traits through the gut microbiome. Using 170 mother-child pairs, PM 10 and NO 2 exposure levels during pregnancy (1st, 2nd, and 3rd trimesters) and annual residential PM 10 levels at age 2, 4, and 6 years were estimated. Autistic traits and gut microbiome were assessed at age 6 years. The associations of PM 10 or NO 2 exposure, gut microbiome composition, and autistic traits were explored, and mediation analyses of statistically significant findings were also conducted. Exposure to PM 10 during the 1st trimester of pregnancy was associated with increased autistic traits (10.6% change per interquartile range (IQR) increase, 95% confidence interval [CI]: 1.1, 21.0) and with Proteobacteria relative abundance at age 6 years (66.9% change per IQR increase, 95% CI: 21.3, 129.8). First trimester NO 2 exposure was associated with autistic traits (12.1% change, 95% CI: 0.1, 25.5) and Proteobacteria relative abundance at age 6 years (48.1% change, 95% CI: -0.1, 119.6). Proteobacteria relative abundance was related to autistic traits (4.4% change per 2-fold increase, 95% CI: 1.3, 7.5). Relations between PM 10 or NO 2 exposure during the 1st trimester and autistic traits at age 6 years were partially mediated by Proteobacteria (proportion mediated 23.2%, p = 0.01 and 16.7%, p = 0.06; respectively). PM 10 and possibly NO 2 exposure during early pregnancy may affect autistic traits at age 6 years through the alteration of Proteobacteria abundance.