Algae serve humans in many ways from inexpensive diatomaceous earth in cleansers and filters to foods. Students observe algal cells to learn about cell structure and function, and certain algae produce agar for use in microbiology studies. Seaweed has been part of Asian diets for centuries, and it has increasingly become part of diets in other parts of the world. Seaweeds sold as food are the following: nori, kelp, alaria, dulse, and digitata. These items are rich in protein and important sources of amino acids that are usually low in other common foods. Seaweeds also provide β-carotene and B-complex vitamins plus a variety of essential macro- and micronutrients: calcium, chloride, potassium, sodium, phosphorus , magnesium, iron, zinc, copper, manganese, iodine, selenium, molybdenum, and chromium.

The food industry uses alginate and carrageenans extracted from seaweeds to add consistency to packaged foods. Alginate thickens puddings, yogurts, sauces, gravies, and syrups, and personal care products such as lotions and toothpaste. The paper and textile industry uses alginate to absorb water generated during production steps, and the drug industry uses it as an inert (inactive) ingredient in drugs. For example, the inert portions of tablet and capsulated drugs contain alginates from red and brown algae. Alginate and carrageenans are safe to consume when used in foods and drugs.

Kelp has been used as protection against radioactive iodine 131 released in small amounts each day from nuclear reactors. It serves this medical purpose because kelp contains a high concentration of iodine 127, which when ingested may block the body's absorption of iodine 131. The University of Delaware oceanographer Geaorge Luther said in 2008 to the campus newspaper U Daily, "Brown kelp has 1,000 times more iodine than what is in the sea, and it is always taking it on." Since the Middle Ages, eating kelp has been used as a cure for goiter, an enlargement of the thyroid gland due to lack of iodine in the diet.

Seaweed has also gained acceptance in Western countries as a fertilizer for home gardens and in agriculture. Coastal populations outside the United States have long used dried seaweed as a fertilizer and for soil conditioning because it is a good source of nutrients for plants.

A new industry based on the harnessing of algae as an energy source for human use has been growing. This invention, called a biocell, generates energy from biological activities such as photosynthesis rather than chemical reactions. In 2007, the San Francisco Chronicle reporter David R. Baker wrote of one of the new enterprises pursuing algae energy: "The algae beneath Harrison Dillon's microscope could one day fuel your car. Dillon's Menlo Park [California] company, Solazyme, has tweaked the algae's genes to turn the microscopic plant into an oil-producing machine. If everything works... vats of algae could create substitutes for diesel and crude oil." So far, algae biocells have been made to generate small electrical outputs, enough to power a hand calculator, for instance. Larger outputs of energy from algae may be part of future energy production.