The red algae, or Rhodophyta ( /roʊˈdɒfɨtə/ or /ˌroʊdəˈfaɪtə/; from Greek: ῥόδον (rhodon) = rose + φυτόν (phyton) = plant, thus red plant), are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species; more detailed counts indicate ~4,000 in ~600 genera (3,738 marine spp in 546 genera and 10 orders (plus the unclassifiable); 164 freshwater spp in 30 genera in 8 orders).

The red algae form a distinct group characterized by the following attributes: eukaryotic cells without flagella and centrioles, using floridean starch as food reserve, with phycobiliproteins as accessory pigments (giving them their red color), and with chloroplasts lacking external endoplasmic reticulum and containing unstacked thylakoids.  Most red algae are also multicellular, macroscopic, marine, and have sexual reproduction.

Many of the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse (Palmaria palmata) and laver (nori/gim) are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans and other food additives. 

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths just past this range are called infrared (below red) and cannot be seen by the naked eye although they can be sensed as heat. In some cultures, certain shades of red are used to symbolize anger or aggression.

It is the color of blood when it is exposed to sunlight. In western countries, it is most commonly referred to as a color of evil, love, and sometimes, though rarely, happiness.

The word red comes from the Old English rēad. The word can be further traced to the Proto-Germanic rauthaz and the Proto-Indo European root reudh-. In Sanskrit, the word rudhira means red or blood. In the English language, the word red is associated with the color of blood, certain flowers (e.g., roses), and ripe fruits (e.g., apples, cherries). Fire is also strongly connected, as is the sun and the sky at sunset. Healthy, light-skinned people are sometimes said to have a "ruddy" complexion (as opposed to appearing pale). After the rise of socialism in the mid-19th century, red was used to symbolize and describe revolutionary movements.