Tomatoes are one of the most significant fruit and vegetable crops in Nigeria. This could be ascribed to the fact that it helps many farmers support their way of life and improve their financial status. However, due to ineffective production management techniques, restrictions on the supply of pesticides and fungicides, access to information, market fluctuations, and crop shelf life, the yield of tomatoes is low. There is a need for powerful institutions like agricultural cooperatives to increase tomato yield. Agricultural cooperatives have been promoted in Nigeria as an agricultural development strategy that will increase crop yield and farmer income. Therefore, this study investigates the impact of membership in agricultural cooperatives on the yields of smallholder tomato farmers in rural Nigeria. A multistage sampling procedure is used to collect data for the study. The Endogenous Treatment Regression model was used to analyze the data. According to the findings of the first regression (Probit regression), age, years of formal education, main occupation, years of farming experience, farm size, years of experience in tomato farming, aged dependents, distance to market, access to credit service and access to extension service have a positive and significant impact on the membership of farmers in the agricultural cooperative. The results of the endogenous treatment regression model reveal marital status, age, years of formal education, years of informal education, main occupation, years of farming experience, farm size, years of tomato farming, aged dependents, distance to market, access to credit services, cost of processing, access to extension service and agricultural cooperative membership are all statistically significant variables influencing tomato yield. According to the ATE estimate, the average farmer would produce 9.159 times more when he/she joined an agricultural cooperative. The conditional treatment effect (ATT), which assesses the impact of membership in agricultural cooperatives on tomato yield, was correspondingly approximately 9.447 and statistically significant at 1%. As a result, the typical farmer who is a member of an agricultural cooperative would produce a yield that is 9.447 times more than it would be if he/she was not a member of an agricultural cooperative. The study found that membership in agricultural cooperatives has a positive impact on tomato yield among smallholder farmers in rural Nigeria in contrast to what it would have been in the absence of being a member of agricultural cooperatives after adjusting for both observable and unobserved factors. Tomatoes farmers must be encouraged to join an agricultural cooperative to boost their yield.