Table grape cultivation has substantial economic benefits. Temperature stress often occurs during the fruit-ripening period, causing premature aging of the vine, abnormal softening of fruit, loss of commercial potential, and reduced yield. Temperature stress, therefore, limits the sustainable and stable production of fresh, high-quality grapes. A key way to avoid this temperature stress is to promote early maturation. Studying methods to shorten the fruit growth period and to develop early-maturing grape cultivars provides an important way to optimize the grape-cultivar market structure. In this study, we utilized the grape cultivar Summer Black, known for its extremely short growth period, along with its bud-mutation cultivar Early Summer Seedless, to analyze early-stage fruit development. The cultivar ESS exhibited a significantly greater color change in the peel than SB at 35-42 DAA, indicating that the veraison period is directly associated with a shortened grape growth period. We therefore conducted transcriptomic analysis of berries harvested during this period, identifying 35 key genes that shorten the berry development period. The AGL gene family participates in plant growth and development. Therefore, we examined the roles of the VvAGL members in fruit growth and development. Screening the 94 common DEGs and 34 VvAGL family members identified VvAGL15 (Vitvi13g01861_t001) as a key gene. VvAGL15 is localized to the nucleus in grapes. The relative expression of VvAGL15 was upregulated in the peel and pulp of ESS relative to that of SB. Fluorescence-based qRT-PCR revealed that VvAGL15 was highly expressed in both the peel and pulp of ESS grapes. Pulp VvAGL15 expression was high at 27 to 42 DAA in both cultivars, indicating that VvAGL15 is important during early berry development, especially during the veraison period. Our heterologous expression analysis using tomato plants revealed that VvAGL15 can accelerate tomato fruit development, leading to faster growth, earlier flowering, earlier acid reduction before color transition, and earlier ripening. These findings provide a theoretical basis for promoting the early ripening of grapes and other crops by enhancing VvAGL15 expression.