This study aimed to investigate the inhibitory effects of Sargassum polycystum extract against Phytophthora palmivora in vitro. The study utilized a quantitative-experimental research design. Quantitative research involves collecting numerical data for statistical hypothesis testing, while the experimental method determines the causality by manipulating an independent variable to observe its effect on a dependent variable (Ghanad, 2023). Seaweed extraction was initially performed to obtain the targeted bioactive compounds particularly flavonoids and phenols, which are associated with antifungal activity. The sample of the collected crude extract was subjected to phytochemical testing to verify the presence of these compounds. Following the extraction process, the extract was evaluated using the Kirby-baeur disc diffusion assay. Four treatments of the seaweed extract (5 μL/disc, 10 μL/disc, 15 μL/disc, 20 μL/disc) were applied to inoculated agar plates and incubated for approximately 6 days. The zones of inhibition were then measured in millimeters (mm) using a caliper. The phytochemical test result indicated a weakly positive reaction for both compounds. In the in-vitro assay, measurable inhibition zones revealed that the 15 μL/disc treatment produced the highest mean zone of inhibition (8.70 mm), followed by the 20 μL/disc treatment (6.84 mm), demonstrating a concentration-dependent effect. It is concluded that Sargassum polycystum extract exhibits minimal but measurable antifungal activity against Phytophthora palmivora; therefore, further research using higher extract concentrations is recommended to further evaluate its antifungal potential.