In rice-based cropping system of northern Bangladesh, mustard planting is delayed due to late harvest of monsoon rice (Transplanted Aman rice; T. Aman rice). The late sown mustard is often exposed to high-temperature stress during the reproductive stage, which causes a significant decrease in seed yield. Most of the farmers plant a low yielding local mustard variety ‘Tori-7’, which is sensitive to high-temperature stress. The Oil Seeds Research Centre of Bangladesh Agricultural Research Institute has recently released short duration and high-yielding mustard varieties for the rice-based cropping system of northern Bangladesh to tackle this issue. However, the performance of these varieties in the target cropping system under late sown conditions (together with exposure to heat stress) has not been evaluated yet. Therefore, this two-year field experiment was conducted to assess the performance of newly released mustard varieties ‘BARI Sarisha-14’ and ‘BARI Sarisha-15’ in comparison with the local check variety ‘Tori-7’ sown at five planting dates (viz. October 21, November 1, November 10, November 20, and November 30). The delay in sowing shortened time (days) to flowering and maturity, regardless of the variety. Similarly, plant height, branches/plant and total dry matter accumulation of the tested varieties were also reduced upon delay in planting. The greatest seed yield was observed in varieties ‘BARI Sarisha-15’ and ‘BARI Sarisha-14’ when sown on October 21. These two varieties produced fair seed yield when sown at November 20. However, the local variety ‘Tori7’ yielded the least across all the planting dates. Therefore, the varieties ‘BARI Sarisha-14’ and ‘BARI Sarisha-15’ may be planted in the rice-based cropping system of northern Bangladesh after harvest of T. Aman rice to get maximum seed yield.