Abstract Seed germination is a vital process in seedling stage. The delayed seed germination and irregular emergence frequently occur in watermelon. It is critical to exploit technologies for promoting seed germination. This study aims to explore the effects of slightly acidic electrolyzed water (SAEW) on the seed germination of watermelon. The results showed that SAEW significantly improved the germination indexes and caused a mass of changes to nutrient substances and enzymatic activities in watermelon seeds. In addition, SAEW treatment decreased abscisic acid (ABA) contents, while increased gibberellin (GA) contents of watermelon seeds by regulating the gene expressions of metabolic related enzymes. Thereinto, SAEW remarkably upregulated the transcriptional level of CYP707A1 and CYP707A2 involved in ABA catabolism, which inhibited the accumulation of ABA. Moreover, SAEW could inhibit reactive oxygen species (ROS) accumulation in seeds and improve the seed germination which was identical to the effect of DMTU (dimethyl thiourea, ROS inhibitor) on the seed germination. In addition, the expressions of RbohC and RbohD involved in ROS accumulation were notably depressed by SAEW. This study demonstrated that exogenous SAEW could inhibit the accumulation of ABA content and decrease the accumulation of ROS, which finally improves the germination ability of watermelon seeds.