No study has been undertaken in Washington state to determine if there is an association between agricultural fairs and Shiga toxin-producing Escherichia coli (STEC) infections, despite a fair-associated outbreak in 2015 that sickened 60 people. The main objective of this study was to determine if there is an increased risk of reported STEC infections within seven days of a county fair in Washington state. A case-crossover study design utilizing conditional logistic regression was performed to estimate the odds ratio (OR) of STEC infection within seven days of a county fair. Across all counties, our model estimated a 31% (95% CI: 2-66%) increase in the odds of a reported case of STEC during and within seven days of a county fair. By age group, a statistically significant OR was found among 10- to 19-year-olds who had an estimated 135% (95% CI: 33-315%) increase in the odds of STEC during and within seven days of a county fair. This study suggests that county fairs are a risk factor for STEC infections in Washington state and highlights the need for continued public health measures to reduce transmission risk at these events.