The use of phytoregulators and phytoprotectants in seed treatment may promote greater tolerance to saline stress. Thus, our objective was to evaluate the effects of seed treatment with phytoregulators and phytoprotectants on the emergence, establishment, development, and quality of watermelon cultivar Crimson Sweet seedlings in biosaline water (BW). The study design was completely randomized, with a 2 × 7 factorial scheme, the first factor consisting of two types of water (drinking water and biosaline water), and the second factor comprising three phytoregulators (salicylic acid, putrescina and gibberellic acid), three phytoprotectors (naphthalic anhydride, thiamethoxam and acibenzolar-S-methyl), in addition to the control treatment. The seedlings were evaluated at 14 days after sowing. Fresh vegetable material was collected to determine the levels of sugars and proteins in the tissues. The salinity of the water affected the development of watermelon seedlings, but it was observed that some evaluated products allow a reduction of the damages caused by the stress. Seedlings were more vigorous when treated with thiamethoxam, even when submitted to biosaline water, compared to control in normal water. Thus, seed treatment with thiamethoxam provides a higher quality of watermelon cultivar Crimson Sweet seedlings in biosaline water.