Bioactive seaweed substances are a group of chemical components extracted from seaweed biomass. There is a long history of the applications of bioactive seaweed substances in functional food products. Seaweed-derived hydrocolloids have long been used as food ingredients. In particular, the unique biophysical properties of alginate and carrageenan are highly valuable in the development of functional food products. As food ingredients, the applications of alginate and carrageenan are based on three main properties, i.e., thickening, gelling, and film forming. In particular, the unique gelling abilities at low temperature alongside good heat stability make alginate ideal for use as thickeners, stabilizers, and restructuring agents. Recently, alginate is increasingly used in a myriad of newer applications, from encapsulating active enzymes and live bacteria to acting as the carrier for protective coating of prepacked, cut, or prepared fruits and vegetables. With novel chemical and biological modifications to alter its structures and properties, there are possibilities of novel applications of specific alginates in the food industry that have high bioactivities at low concentrations. In general, seaweed-derived functional foods can provide health benefits by reducing the risk of chronic diseases and enhancing the ability to manage chronic diseases, thus improving the quality of life. They can also promote growth and development and enhance performance. Marine nutraceuticals can be derived from the many varieties of seaweeds. For example, fucoidan is a complex fucose-rich sulfated carbohydrate, which can be extracted from brown seaweeds. This biologically active carbohydrate has been shown to inhibit a wide range of cancer cell lines, and studies in mice indicate that anticancer effects are also seen in vivo. As a marine nutraceutical product, fucoidan has been used in many health products and is important for its high bioactive properties, for example, antibacterial, anticoagulant, antiviral, antitumor, etc. Seaweeds and marine microalgae are natural sources for β-carotene, astaxanthin, and eicosapentaenoic acid that have high bioactivities and are an important part of nutraceutical products, playing a significant role in a number of aspects of human health. Looking into the future, bioactive seaweed substances are valuable in a group of industries including pharmaceuticals, nutraceuticals, functional foods, biomedical materials, cosmetics, and fertilizers, in addition to the traditional applications in textiles, chemicals, and environmental protection. Once extracted, separated, and purified, the various types of bioactive seaweed substances can be screened for their bioactivities and utilized in an appropriate application. Modern technologies can allow the separation of these native ingredients into purified chemical compounds. Once structurally characterized and functionally assessed, the highly active compounds can be applied in marine drugs, whereas those with lower activities can be used in herbal medicines, nutraceuticals, biomedical materials, functional foods, and other applications based on the level of their bioactivities.