The Phaeophyceae (which means brown algae) is a class of primarily marine algae with a worldwide distribution, abundant in any coastal rocky area, especially in cold waters. They are significant primary producers in certain ecosystems, as they can form true underwater forests with individuals exceeding a hundred meters in length.

These algae typically exhibit brown colorations due to fucoxanthin, a brown pigment that masks the green colour of chlorophyll. Within this group, there are forms ranging from filamentous structures with a very simple and microscopic structure to large algae with a relatively complex morphology and specialized organs for specific tasks, these being the algae with the most complex morphology. Their appearance varies from crusts or filaments to fans, branched ribbons, bushes, or structures resembling a plant with stems, roots, and leaves. Some species have gas vesicles that are used as flotation organs. They have varied life cycles, typically with various morphologically distinct generations necessary to complete the cycle. Spermatozoa with two unequal flagella are characteristic.