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Nematicidal activity of four species of marine algae Ulva fasciata Delile (UF) (green algae), Corallina mediteranea, Corallina officinalis (red algae) and Spirulina platensis (blue-green algae) at concentrations of (125, 250, 500 and 1000 µg/ml) for aqueous and ethanolic extracts were investigated against the knot-root nematode, Meloidogyne incognita in laboratory and greenhouse on tomato plants. In laboratory experiments, all tested algae caused significant mortality of the second larval stage (J2). It was, also, noticed that the tested algae reduced egg hatching. Ethanol extract of all algae possesses highest nematicidal activity than water extract. U. fasciata, ethanolic extracts showed the highest nematicidal activity among the tested algae in vitro. The results of greenhouse experiment clarified that the tested ethanolic extracts of all algae, reduced numbers of root galls, egg masses and population of nematodes of tomato plants at concentration 1000 mg/kg soil compared to the inoculated control. U. fasciata was the most effective treatment in reducing root galls, egg masses and final population of the nematode except for oxamyl. Results revealed that all tested treatments increased various measures of plant growth characters; fresh shoot and root weights and lengths as well as reduced the root-knot infestation. Among all of the tested algae, U. fasciata followed by C. officinalis and C. mediteranea were the most effective treatments to increase both fresh shoot & root weights and lengths. None of the algae was phytotoxic at the tested treatments. A chemical constituent of U. fasciata was recognized by GC-MS. GC-MS analysis, exhibited the presence of organic component such as bis(2-ethylhexyl) phthalate which ranked the first with 63.75%, followed by diethyl phthalate (18.46%). Results stated that marine algae and especially, U. fasciata exhibited significant nematicidal activity in-vitro and in-vivo experiments and could be considered as useful natural namaticidal agents.