Long-term air pollution exposure can have detrimental effects on respiratory, cardiovascular, and cerebrovascular health, increasing mortality risk. Ultrafine particles (UFPs) are nanoscale air pollution particles with aerodynamic diameters under 100 nm. Growing evidence suggests that UFPs can deteriorate brain health by crossing the blood-brain barrier that protects the brain from toxins and pathogens. Historically, people of color and low income communities have been more likely to be exposed to higher concentrations of air pollution, but disparities in UFP exposure specifically have not been studied. This study used linear regression modeling to estimate the association of UFP exposure with race-ethnicity and income in the greater Seattle area. We observed that 83% of the census block groups in the study area were predominantly non-Hispanic white, and household incomes ranged from $9,000 to $250,000 with a median income of $71,000. Estimates of annual average UFP concentrations were modeled from mobile monitoring measurements during the year 2019. UFP concentrations ranged from 2,000 to 14,000 pt/cm3 with a mean of 6,000 pt/cm3. We found that UFP concentration decreased by 177 pt/cm3 (95%CI: 160, 194) for every 10% increase in household income. For every 10 percentage point increase in non-Hispanic white population, UFP concentration decreased by 449 pt/cm3 (95%CI: 410, 488). These results indicate disparities in UFP exposure due to both race-ethnicity and income in the Seattle area. Further analyses investigated this relationship in detail, by specific income and racial-ethnic groups, and compared the relative effects of race-ethnicity and income on UFP exposure. This study can help inform future studies on UFPs and public policies that promote healthier, more equitable communities.