Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is a devastating disease of rapeseed/canola that causes significant seed yield loss, reduced oil content, and quality. Lack of complete immune genotypes and polygenic resistance between host and pathogen often impedes the development of functional molecular markers and gene identification to enable SSR resistance breeding. However, genomics-assisted breeding approaches such as genome-wide association (GWA) mapping and genomic prediction (GP) are considered most promising for the genetic improvement of complex traits over classical breeding. Therefore, the objective of this study was to perform GWA mapping and GP in a diverse rapeseed/canola panel using ~24,000 to ~28,000 single nucleotide polymorphisms (SNPs) under field and greenhouse environments. Extensive phenotyping against S. sclerotiorum infection revealed few lines had promising resistance at seedling and adult stages in both environments. Adult plant resistance (APR) was characterized in four field environments by recording four traits and found strong associations among them. GWA models using the four traits identified 133 SNPs and 69 putative candidate genes associated with APR. The predictive ability (PA) ranged from 0.41-0.64 depending on trait specifications. For seedling resistance (SR) under a greenhouse environment, multiple GWA models using multiple traits detected 219 SNPs. Multiple GP models resulted in 0.45-0.68 PA for these traits. Association analyses for APR under controlled environments using five traits identified 37 and 50 significant SNPs in spring (SP) and semi-winter & winter populations (SWP), respectively. GP analyses revealed 0.48-0.60 and 0.10-0.19 PA in SP and SWP, respectively. Based on the GWA results collected from all experiments, we detected previously mapped overlapping genomic regions as well as new regions on chromosome A09 (33.34-39.13 Mb), C02 (59.17-62.79 Mb), and C6 (32.24-37.67 Mb). These findings would provide exciting opportunities to narrow the genomic regions to guide map-based cloning of SSR resistance genes to assist in future marker-assisted selection. Moreover, we have achieved a medium to high PA by implementing GP. Our study concludes that GWA mapping and GP hold promise to lead a step forward towards the genomics-assisted SSR resistance rapeseed/canola breeding that would help to achieve rapid gains from the selection.