Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodelling complexes are multi-protein machineries that control gene expression by regulating chromatin structure in eukaryotes. However, the full subunit composition of SWI/SNF complexes in plants remains unclear. Here we report that in Arabidopsis thaliana, two homologous glioma tumour suppressor candidate region domain-containing proteins, named BRAHMA-interacting proteins 1 (BRIP1) and BRIP2, are core subunits of plant SWI/SNF complexes. brip1 brip2 double mutants exhibit developmental phenotypes and a transcriptome remarkably similar to those of BRAHMA (BRM) mutants. Genetic interaction tests indicated that BRIP1 and BRIP2 act together with BRM to regulate gene expression. Furthermore, BRIP1 and BRIP2 physically interact with BRM-containing SWI/SNF complexes and extensively co-localize with BRM on chromatin. Simultaneous mutation of BRIP1 and BRIP2 results in decreased BRM occupancy at almost all BRM target loci and substantially reduced abundance of the SWI/SNF assemblies. Together, our work identifies new core subunits of BRM-containing SWI/SNF complexes in plants and uncovers the essential role of these subunits in maintaining the abundance of SWI/SNF complexes in plants.