In pat-2 parthenocarpic tomatoes (Solanum lycopersicum L.), inhibition of seed production prevents their breeding and development. Low seed set is thought to be caused by a pleiotropic effect of the pat-2 gene because the number of seeds from pat-2 parthenocarpic tomato cultivars is generally low; however, research on the relationship between low seed set and the pat-2 gene is lacking. In this study, to clarify the effect of the pat-2 gene on seed production and develop a method for promoting seed production in pat-2 parthenocarpic tomatoes, we examined the relationship between the pat-2 gene and low seed set and the effect of aminoethoxyvinylglycine (AVG), an indole-3-acetic acid (IAA) biosynthesis inhibitor, on seed production in ‘Renaissance’, a pat-2 parthenocarpic tomato cultivar. In an F2 population derived from a cross between ‘Micro-Tom’ and ‘Renaissance’, the number of seeds in plants homozygous for the pat-2 allele was significantly lower than that in heterozygous plants and that homozygous for the Pat-2 allele. This indicated that low seed set was genetically associated with the pat-2 allele, and that low seed set was unavoidable when using the pat-2 gene for breeding of parthenocarpic tomato cultivars. Therefore, it is important to develop a method to promote seed production in pat-2 parthenocarpic tomatoes. In this study, we examined the effect of AVG on seed production in ‘Renaissance’. We found that treatment with 30 μM AVG significantly increased the number of seeds in ‘Renaissance’. In addition, when we treated flower clusters with AVG solution at the first flower anthesis, the effect of treatment with 30 μM AVG on increasing seed set was highest for the flower in the second position in a cluster, which suggests the effect of AVG on promoting seed production changed depending on the stage and/or position. Our results showed that low seed set was genetically associated with the pat-2 allele and that AVG is an effective tool to promote seed production in pat-2 parthenocarpic tomatoes. Based on these results, research progress on improving seed production in parthenocarpic tomatoes with the pat-2 gene is expected to accelerate in the future.