Seaweeds have beneficial nutritional and medicinal properties. Several studies have examined the polysaccharides found in the extracts of Porphyra yezoensis (PPY), although the effects of particular proteins have not been reported, and peptides from the marine alga PPY function in antitumor cell signaling, although the precise mechanism is not well understood. Apoptosis plays an important role in cell death, which affects cell proliferation. Generally, regulation of apoptosis requires participation of the p53 and Bcl-2 family by the mammalian target of rapamycin (mTOR) pathway, which is activated in a variety of malignant cancers. Autophagy is another signaling pathway that leads to degradation of cellular components by lysosomal activity, and the relationship between autophagy and cancer has been of interest for several years. The present study investigated mTOR pathway activation in MCF-7 cells treated with 500 ng PPY for 24 h by assessing LC3 as a monitor of autophagy. We observed that the p53/NF-κB and mTOR pathways were affected by PPY, which contributes to our understanding of the functional relationship between the Bcl-2 family and mTOR under apoptotic conditions in MCF-7 cells.