Plant architecture is the key factor affecting overall yield in many crops. The genetic basis underlying plant architecture in rapeseed (Brassica napus), a key global oil crop, is elusive. We characterized an ethyl methanesulfonate (EMS)-mutagenized rapeseed mutant, sca, which had multiple phenotypic alterations, including crinkled leaves, semi-dwarf stature, narrow branch angles and upward-standing siliques. We identified the underlying gene, which encodes an Aux/IAA protein (BnaA3.IAA7). A Gto-A mutation changed the glycine at the 84th position to glutamic acid (G84E), disrupting the conserved degron motif GWPPV and reducing the affinity between BnaA3.IAA7 and TIR1 (TRANSPORT INHIBITOR RESPONSE 1) in an auxin dosage-dependent manner. This change repressed the degradation of BnaA3.IAA7 and therefore repressed auxin signaling at low levels of auxin that reduced the length of internodes. The G84E mutation reduced branch angles by enhancing the gravitropic response. The heterozygote +/sca closely resembled a proposed ideal plant architecture, displaying strong yield heterosis through single-locus overdominance by improving multiple component traits. Our findings demonstrate that a weak gain-of-function mutation in BnaA3.IAA7 contributes to yield heterosis by improving plant architecture and would be valuable for breeding superior rapeseed hybrid cultivars and such a mutation may increase the yield in other Brassica crops.