Chitinase plays an important role in plant resistance against chitin containing pathogens by degrading chitin. Clubroot, caused by Plasmodiophora brassicae, is a major disease in cruciferous crops worldwide, including vegetable and rapeseed. Chitin is the main component of P. brassicae spore cell walls, chitinases can enhance the plant’s defense system as they act on chitin. However, the function of chitinases in P. brassicae has not reported. Here, we found that chitin is the functional component of P. brassicae by wheat germ agglutinin staining and commercial chitinase treatment . Chitin extracted from resting spores of P. brassicae was characterized by infrared spectroscopy, X-ray diffraction and thermo gravimetry analysis.The chitinase PbChia1 was identified by chitin pull-down assays combined with LC-MS/MS method. We also showed that PbChia1 was a typical secreted chitinase, could bind to chitin and showed chitinase activity in vitro, significantly degrade the number of resting spores of P. brassicae and significantly relieve the severity of clubroot symptom, the biocontrol effect was 61.29%. Overexpression of PbChia1 in Arabidopsis thaliana resulted in susceptibility to P. brassicae, increase host survival rate and seed yields, and transgenic plants enhanced in chitin-triggered reactive oxygen species burst, MAPK activation and expression of immune-related genes. PbChia1 transgenic plants showed resistance to other pathogens, such as biotrophic bacteria Pst DC3000, necrotrophic fungi Sclerotinia sclerotiorum 1980 and Rhizoctonia solani. The findings indicate that chitinases PbChia1 is a candidate gene with potential application for broad spectrum disease resistance in breeding plants to prevent clubroot.