Rhodophyta (red algae) has been recognized as a rich source of natural compounds with a wide range of medicinal and biological properties, including anti-oxidants, anti-proliferative, anti-tumor, anti-viral, and anti-coagulant properties. The main goal of this study is to structurally characterize and assess the cytotoxicity of both ethanolic and chloroform extracts of Sarconema filiforme and Laurencia obtusa red seaweed collected from the Suez Canal coasts, Egypt. Extracts of S. filiforme and L. obtusa were prepared and characterized using GC/MS analysis and MTT assay was used to examine their toxicity against human breast cancer (MCF-7 and MDA-MB-231 cells) and lung adenocarcinoma (A549 cells) cell lines. Using GC/MS  analysis, a total of 34 and 29 different compounds were identified in the extracts of S. filiforme and L. obtusa, respectively. The identified components include alkanes, phenol derivatives, benzoic acid esters, fatty acids, fatty acid esters, terpenes, fatty acyls, steroids, and prostaglandin. The most abundant constituents in both algae species are cholesterol and diisooctyl phthalate. The in vitro assays revealed a mild to moderate cytotoxicity of the two crude extracts of both species on MCF-7, MDA-MB-231, and A549 cells. Based on GC/MS analysis, it is obvious that S. filiforme and L. obtusa extracts contain various biologically active compounds with potential antimicrobial, antioxidant, antiviral, anti-inflammatory and antitumor activities. Further bioguided fractionations are recommended to identify and isolate antitumor active compounds of both extracts.