Organ size is a major agronomic trait that determines grain yield and biomass production in crops. However, the molecular mechanisms controlling organ size, especially in legumes, are poorly understood.

• Using forward genetic approaches in a Tnt1 insertion mutant population of the model legume Medicago truncatula, we identified SMALL LEAF AND BUSHY1 (SLB1), which is required for the control of organ size and lateral branching.

• Loss of function of SLB1 led to reduced leaf and flower size but increased lateral branch formation in M. truncatula. SLB1 encodes an F‐box protein, an orthologue of Arabidopsis thaliana STERILE APETALA (SAP), that forms part of an SKP1/Cullin/F‐box E3 ubiquitin ligase complex. Biochemical and genetic analyses revealed that SLB1 controls M. truncatula organ growth and lateral branching by modulating the stability of BIG SEEDS1 (BS1). Moreover, the overexpression of SLB1 increased seed and leaf size in both M. truncatula and soybean (Glycine max), indicating functional conservation.

• Our findings revealed a novel mechanism by which SLB1 targets BS1 for degradation to regulate M. truncatula organ size and shoot branching, providing a new genetic tool for increasing seed yield and biomass production in crop and forage legumes.