Abstract: DNA methylation confers epigenetic regulation on gene expression and thereby on various biological processes. Tomato has emerged as an excellent system to study the function of DNA methylation in plant development. To date, regulation and function of DNA methylation maintenance remains unclear in tomato plants. Here, we report the critical function of tomato (Solanum lycopersicum) Methyltransferase 1 (SlMET1) in plant development and DNA methylome and transcriptome regulation. Using CRISPR‐Cas9 gene editing, we generated slmet1 mutants and observed severe developmental defects with a frame‐shift mutation, including small and curly leaves, defective inflorescence, and parthenocarpy. In leaf tissues, mutations in SlMET1 caused CG hypomethylation and CHH hypermethylation on a whole‐genome scale, leading to a disturbed transcriptome including ectopic expression of many RIN target genes such as ACC2 in leaf tissues, which are normally expressed in fruits. Neither the CG hypomethylation nor CHH hypermethylation in the slmet1 mutants is related to tissue culture. Meanwhile, tissue culture induces non‐CG hypomethylation, which occurs more frequently at gene regions than at TE regions. Our results depict SlMET1‐ and tissue culture‐dependent tomato DNA methylomes, and that SlMET1 is required for maintaining a normal transcriptome and normal development of tomato.