Polyculture of black tiger shrimp Penaeus monodon, mud crab Scylla paramamosain, and red seaweed Gracilaria tenuistipitata, were applied in improved extensive ponds in the Vietnamese Mekong delta to evaluate production and financial efficiency. The trial consisted of two treatments: (1) non-feed supplement, in which shrimp consumed only natural food in the culture pond; and (2) feed supplement, in which shrimp received commercial feed (50% feeding rate). Treatments were in triplicate; ponds featured natural occurrence of red seaweed. Postlarvae of shrimp and crab were stocked at densities of 4 ind./m2 and 0.15 ind./m2, respectively, and culture for six months. Results showed that shrimp receiving feed supplement demonstrated much better (P < 0.05) growth and survival than those without feed supplementation. Shrimp yield was significantly higher (P > 0.05) in the feed supplement pond (348.9 kg/ha) than in the non-feed supplement pond (169.2 kg/ha). Crab production was similar between the two treatments (P > 0.05), varying from 88.1 to 92.4 kg/ha. Production cost (456.2 US$/ha) in the non-feed supplement pond was significantly lower than that in the feed supplement pond (944.8 US$/ha), but the former produced less profit (2,230.9 US$/ha) than the latter (4,169.2 US$/ha). Notably, the average benefit–cost ratio in the non-feed supplement group (4.84 times) was relatively higher compared to the feed supplement group (4.35 times). These findings suggest that polyculture of shrimp, crab, and seaweed in the improved extensive pond could be applied either non-feed supplement or feed supplement for shrimp, depending on the financial conditions of the farmers.