Commercially valuable sea cucumbers are potential co-culture species in tropical lagoon environments, where they may be integrated into established aquaculture areas used for seaweed farming. In the current study, wild-caught juvenile sea cucumbers, Holothuria scabra, and red seaweed Kappaphycus striatum were co-cultured on Zanzibar, United Republic of Tanzania. Sea cucumbers (97 g 31 SD, n = 52) were cultured in mesh enclosures at initial cage stocking densities of 124 21 SD and 218 16 SD g m 2 under seaweed culture lines. Over 83 days, individual growth rate (1.6 g d 1 0.2 SD) of sea cucumbers at low stocking density was significantly higher (v2 = 8.292, d.f. = 1, P = 0.004) than at high-stocking density (0.9 g d 1 0.1 SD). Seaweed individual growth rates [6.27 ( 0.3 SE) g d 1] were highest in co-culture with sea cucumber at low density but did not differ significantly from high sea cucumber density or seaweed monoculture treatments (v2 = 3.0885, d.f. = 2, P = 0.2135). Seaweed growth varied significantly (v2 = 35.6, d.f. = 2, P < 0.0001) with sampling period, with the final sampling period resulting in the highest growth rate. Growth performance for seaweed and sea cucumbers (v2 = 3.089, d.f. = 2, P = 0.21 and v2 = 0.08, d.f. = 1, P = 0.777 respectively), did not differ significantly between monoculture and co-culture treatments, yet growth in co-culture was comparable with that reported for existing commercial monoculture. Results indicate H. scabra is a highly viable candidate species for lagoon co-culture with seaweed. Co-culture offers a more efficient use of limited coastal space over monoculture and is recommended as a potential coastal livelihood option for lagoon farmers in tropical regions.