The rise in cancer cases has prompted searching for novel and alternative sources of natural bioactive compounds with antitumor potential. Nearly three-quarters of our planet is covered by the ocean, the habitat of numerous prokaryotic and eukaryotic organisms and sustainable alternative nutrient sources. The marine ecosystem is a rich reservoir of proteins and novel bioactive peptides with diverse biochemical and therapeutic potentials, including antioxidant, anti-inflammatory, and antiproliferative activities. Marine peptides are valuable due to their high stability, bioactivity, and low immunogenicity. This review focused on tracking the recent progress in studying marine-derived peptides for potential cancer treatment. We have highlighted that some of these peptides have progressed to clinical trials in the last 2 decades, while many candidates were discontinued due to failure to exhibit therapeutic-relevant activities. Due to the results from old clinical trials, interest in marine sources for antitumor peptides has dwindled in recent years. We presented other possible limitations in this field and proposed attractive future research prospects. In conclusion, we emphasize the need for increased scientific attention to explore marine organisms’ untapped nutraceutical and bioactive natural products, particularly in uncovering their potential anticancer properties.