Hypertension is a major public health concern that requires continuous pharmacological treatment and lifestyle modification. Watermelon juice contains bioactive compounds, particularly L-citrulline, that may support blood pressure regulation through nitric oxide synthesis and vasodilation. This study aimed to evaluate the effectiveness of watermelon juice in reducing blood pressure among patients with hypertension participating in Prolanis at Pilolodaa Public Health Center, Gorontalo City. A quantitative quasi-experimental study using a non-equivalent control group pretest–posttest design was conducted with 27 respondents, comprising 13 participants in the intervention group and 14 in the control group. The intervention group received watermelon juice alongside their usual antihypertensive medication, while the control group continued usual care. Data were analyzed using the Wilcoxon Signed-Rank Test and Mann–Whitney U test with a significance level of 0.05. The results showed a statistically significant difference between pretest and posttest blood pressure measurements in the intervention group (Z = −2.500, p = 0.012), whereas no significant difference was observed in the control group (Z = 0.000, p = 1.000). The between-group comparison showed no significant difference at baseline (p = 0.793) but a significant difference at posttest (p = 0.017). These findings suggest that watermelon juice may have potential as a complementary dietary intervention for hypertension management. However, the small sample and quasi-experimental design warrant cautious interpretation, and further studies using standardized protocols and direct comparisons of blood pressure changes are needed.