Phytosulfokine-a (PSK-a), a sulfated pentapeptide hormone with the sequence YIYTQ, plays important roles in many aspects of plant growth and development. In this study, we identified a pair of putative precursor genes in soybean, GmPSK?1 and -2, encoding a PSK-like peptide: PSK-?. Similar to PSK-a in amino acid composition, the sequence of PSK-? is YVYTQ, and the tyrosines undergo sulfonylation. Treatment of Arabidopsis seedlings with synthetic sulfated PSK-? significantly enhanced root elongation, indicating that PSK-? might be a functional analog of PSK-a. Expression pattern analysis revealed that the two GmPSK? genes, especially GmPSK?1, are primarily expressed in developing soybean seeds. Heterologous expression of GmPSK?1 under the control of a seed-specific promoter markedly increased seed size and weight in Arabidopsis, and this promoting effect of PSK-? on seed growth was further confirmed in transgenic tobacco constitutively expressing GmPSK?1. Cytological analysis of transgenic Arabidopsis seeds revealed that PSK-? promotes seed growth by inducing embryo cell expansion. In addition, transcriptome analysis of GmPSK?1-expressing Arabidopsis seeds suggested that PSK-? signaling may regulate cell wall loosening to promote cell expansion. Overall, our results shed light on the mechanism by which PSK-? promotes seed growth, paving the way for the use of this new peptide for biotechnological improvement of crop seed/grain size and yield. Overall design: Seed (10 DAP) transcriptomes of OLEOp:GmPSK?1 and OLEOp:GmPSK?1tc (control) were generated by deep sequencing using Illumina Hiseq 2500. RNA isolated from three individual lines of each genotype were pooled together for sequencing.