SUMMARY: Fruit size and shape are controlled by genes expressed during the early developmental stages of fruit. Although the function of ASYMMETRIC LEAVES 2 (AS2) in promoting leaf adaxial cell fates has been well characterized in Arabidopsis thaliana, the molecular mechanisms conferring freshy fruit development as a spatial–temporal expression gene in tomato pericarp remain unclear. In the present study, we verified the transcription of SlAS2 and SlAS2L, two homologs of AS2, in the pericarp during early fruit development. Disruption of SlAS2 or SlAS2L caused a significant decrease in pericarp thickness as a result of a reduction in the number of pericarp cell layers and cell area, leading to smaller tomato fruit size, which revealed their critical roles in tomato fruit development. In addition, leaves and stamens exhibited severe morphological defects in slas2 and slas2l single mutants, as well as in the double mutants. These results demonstrated the redundant and pleiotropic functions of SlAS2 and SlAS2L in tomato fruit development. Yeast two‐hybrid and split‐luciferase complementation assays showed that both SlAS2 and SlAS2L physically interact with SlAS1. Molecular analyses further indicated that SlAS2 and SlAS2L regulate various downstream genes in leaf and fruit development, and that some genes participating in the regulation of cell division and cell differentiation in the tomato pericarp are affected by these genes. Our findings demonstrate that SlAS2 and SlAS2L are vital transcription factors required for tomato fruit development. Significance Statement: Understanding early fruit development in tomato is critical since the size and shape of a fruit is largely decided in the early stages. Our previous work identified hundreds of transcription factors specifically expressed in the pericarp of 0–5 days post anthesis fruits in tomato that are likely involved in this complex process. ASYMMETRIC LEAVES 2 (AS2) belongs to the LATERAL ORGAN BOUNDARIES family, and its role in promoting leaf adaxial cell fates in Arabidopsis has been well characterized. SlAS2 and SlAS2L, two homologs of AS2 in tomato, are specifically expressed in the ovary wall/pericarp at early fruit developmental stage, but their functions in fleshy fruit development are still unclear. In the present study, we found that knockout mutations in SlAS2 or SlAS2L caused significantly reduced pericarp thickness, leading to smaller mature fruit size. Histocytological observation of the pericarp revealed that disruption of SlAS2 or SlAS2L inhibited both cell division and expansion, leading to the observed thinner pericarp. Additionally, leaf and stamen defects in single and double mutants suggested that SlAS2 and SlAS2L have redundant functions in plant development. Both SlAS2 and SlAS2L interact with SlAS1 in vitro, which is consistent with Arabidopsis AS2. Molecular analyses revealed that genes regulated by SlAS2 and SlAS2L in leaf and fruit development are different, and were related to cell division and differentiation processes during tomato pericarp development. Our results confirm the vital roles of SlAS2 and SlAS2L in regulating tomato fruit development. [ABSTRACT FROM AUTHOR]