We investigated the role of a brown seaweed alga, Sargassum tenerrimum (J. Agardh), against Dysdercus cingulatus (Fab.), which is an economically important cotton pest in many parts of Asia. Sargassum tenerrimum has been used as a source of fertilizers and drugs, but there is no information available in the literature about its use in ecofriendly pest management. The impact of benzene (BN), chloroform (CH), and a mixture of benzene and chloroform (BNCH) extracts, as well as two chromatographic fractions of BNCH (FR1 and FR2) were investigated for their insecticidal and ovipositional properties against D. cingulatus. The BE extract showed the best insecticidal activity (LC50 = 0.009%), higher than BNCH (LC50 = 0.021%) and CH extracts (LC50 = 0.2481%), and all the extracts reduced the total nymphal developmental period of the pest in a dose-dependent way. Adult longevity of both males and females was reduced by the CH and BE extracts. The CH extract reduced D. cingulatus oviposition, preoviposition, and post-oviposition periods, as well as fecundity. Females emerging from the BE category could not lay eggs. All the extracts significantly reduced both total body protein and genomic DNA content. The BECH fractions, FR1 and FR2, also exhibited nymphicidal activity against the pest. However, these fractions did not significantly alter the nymphal developmental period. Results indicate that these seaweed extracts can be used as biopesticides for pest management.