This study investigates the inhibitory effects of thalli and their extracts of the macroalga Turbinaria ornata on the germination of dinoflagellate cysts, previously isolated from Red Sea surface sediments. The experiments were conducted on cysts of five harmful dinoflagellate species including Alexandrium catenella, Cochlodinium polykrikos, Dinophysis accuminata, Prorocentrum cordatum and Scrippsiella trochoidea. The results showed neither macroalgal thalli nor their extracts had direct impact on the cyst germination of all species. Instead, these macroalgal materials remarkably affected the germling viability and culturability of progeny cells of these cysts. Dry macroalgal thalli exhibited stronger inhibitory effects on germling viability and cell culturability (IC₅₀= 0.235–0.543, 0.385–1.43 mg mL⁻¹, respectively) than fresh thalli (IC₅₀=2.201–4.716, 2.17–7.18 mg mL⁻¹, respectively). The macroalgal ethanol extract was approximately 2-5 times more effective (IC₅₀ = 0.012–0.047 and 0.024–0.089 mg mL⁻¹, respectively) than aqueous extract (IC₅₀ = 0.04–0.1 and 0.054–0.207 mg mL⁻¹, respectively) against the germling viability and vegetative progeny cells of all cyst species. Among different species, A. catenella and C. polykrikos germlings were more sensitive to macroalgal thalli and their extracts than those of S. trochoidea, P. cordatum and D. acuminata. Meanwhile, progeny cells of A. catenella exhibited the highest sensitivity to all macroalgal materials. Our results suggest that the use of T. ornata may be a promising strategy for inhibiting the division of progeny cells of dinoflagellate cysts and impairing the recurrence of HABs in confined coastal areas.