As a renewable resource, seaweeds are abundant in secondary metabolites that have the potential to accelerate metabolic processes in insects, induce toxicity, alter growth and development and delay oviposition and feeding activities. The purpose of the current experiment was to examine the insecticidal activity of brown (Sargassum wightii, Stoechospermum marginatum, Turbinaria conoides) and red algal seaweeds (Gracilaria edulis, Hypnea spicifera, Liagora ceranoides) against diamondback moth under laboratory conditions in Agricultural College and Research Institute, Madurai. The marine algae were collected from Ramanathapuram districts of Tamil Nadu, India. Different extracts of seaweeds were made using the Soxhlet extraction method and were tested for toxicity at various concentrations, including 1%, 2% and 3% against a homogenous culture of second-instar Plutella xylostella larvae. Their effects were compared to those of solvents (Methanol, chloroform, hexane and acetone), a treated group (Azadirachtin 1% at 2 ml/l) and an untreated control under leaf dip bioassay with three replications in Completely Randomized Design. The larval mortality data on the treated larvae were collected systematically, statistically examined and reported. Among different concentrations of tested seaweed extracts, the highest larvicidal activity (83.33%) was observed after 72 h with Sargassum wightii methanolic extracts at 3% concentration and the least larval mortality was observed for Gracilaria edulis acetone extracts at 1% concentration (20.00%). Phytochemical profiling of all methanolic extracts of seaweeds was done by GC-MS/MS analysis. The heat map of the metabolites identified nine metabolites (6-Octadecanoic acid, Papveroline, Paroxypropione, o-Methoxymandelic acid, 11, 14-Eicosadienoic acid, Oxirane, 2-Hydroxy-5-Methyl acetophenone, Alpha-Bromo-gamma-valerolactone, 2-O-Methlyl-d-xylose) in S. wightii with high intensity that were not detected in other species. These metabolites are likely to possess insecticidal property. This study suggests the use of seaweed extracts as botanical pesticides which will be an alternative in the creation of an Integrated Pest Management (IPM) module for the control of diamondback moths.