Cultivated soybeans may lose some useful genetic locus during domestication. Introgression of genes from wild soybeans may broaden the genetic background and improve soybean agronomic traits. Here, through whole-genome sequencing of an RIL population derived from a cross between a wild soybean ZYD7 and a cultivated soybean HN44, and mapping of QTLs for seed weight, we discover that a phosphatase 2C-1 (PP2C-1) allele from wild soybean ZYD7 contributes to the increase of seed weight/size in transgenic plants. The PP2C-1 may achieve this function by enhancing cell size of integument and activating a subset of seed trait-related genes. The PP2C-1 was further found to associate with a transcription factor GmBZR1 and facilitate accumulation of dephosphorylated GmBZR1. In contrast, a PP2C-2 allele with variations of a few amino acids at N-terminus does not exhibit this function. Moreover, the GmBZR1 can promote seed weight/size in transgenic plants. Through analysis of cultivated soybean accessions, we find that 40% of the examined accessions do not have the PP2C-1 allele, suggesting that these accessions can be improved through introduction of the PP2C-1 allele. Our study identifies an elite allele PP2C-1, which can enhance seed weight/size. Manipulation of the allele by molecule-assisted breeding may increase production in soybean and other legumes/crops.