<p>Sclerotinia stem rot (SSR) of canola (<i>Brassica napus</i>), caused by the fungal pathogen <i>Sclerotinia sclerotiorum</i>, is a serious disease of the crop and is present in all major growing areas of the world. Using resistant cultivars is an effective approach to managing the disease. The current study aimed to screen forty double-haploid advanced genotypes along with three commercial canola cultivars for resistance in field conditions in a hot spot region, Gorgan, northern Iran, over two growing seasons of 2019–20 and 2020–21, and under glasshouse conditions. Stems of mature plants and petioles of seedlings were inoculated with agar plugs. Their reaction to SSR was measured using lesion length and lesion phenotype in the field and glasshouse, respectively. The data collected was analyzed using linear mixed models, non-parametric tests, and clustering statistical methods. The results of the field and the glasshouse were compared and integrated. In the field, significant differences were detected among cultivars, between years, and their interaction. Moreover, in the glasshouse, differences among genotypes were significant. No significant correlations were detected between SSR levels in greenhouse and field conditions. Two genotypes, Option500 × RGS3006 and RGS003 × Hyola 308, were identified as highly resistant, making them suitable for breeding programs or release as new cultivars. The resistant genotypes identified in this study have the potential to be cultivated in regions with a history of infection. They can also be combined with other agronomic practices and chemicals to achieve a suitable level of disease control. The distribution of lesion phenotype in seedling stages followed a bell-shaped curve, suggesting a quantitative expression of polygenic inheritance responsible for disease resistance.</p>