Background: In line with our recent discovery of an efficient anticancer thiazolebenzenesulfonamide framework HA15 (1) based on a remarkable endoplasmic reticulum stress inducement mode of action, we report herein a series of innovative constrained HA15 analogs, featuring four types of bicylic derivatives. Results: The structure–activity relationship analysis, using a cell line assay, led us to identify a novel version of HA15: a new benzothiazole derivative (10b) exhibiting important anti-melanoma effect against sensitive and resistant melanoma cells. Meanwhile, compound 10b induced a significant tumor growth inhibition in vivo with no apparent signs of toxicity. Conclusion: These results consistently open new directions to improve and develop more powerful anticancer therapeutics harboring this type of fused framework.