Ochrophytes, also known as heterokontophytes or stramenochromes, are a phylum of algae. They are the photosynthetic stramenopiles, a group of eukaryotes, organisms with a cell nucleus, characterized by the presence of two unequal flagella, one of which has tripartite hairs called mastigonemes. In particular, they are characterized by photosynthetic organelles or plastids enclosed by four membranes, with membrane-bound compartments called thylakoids organized in piles of three, chlorophyll a and c as their photosynthetic pigments, and additional pigments such as β-carotene and xanthophylls. Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such as brown algae and diatoms. They are classified either as phylum Ochrophyta, Heterokontophyta or as subphylum Ochrophytina within phylum Gyrista. Their plastids are of red algal origin.

Ochrophytes are eukaryotic organisms composed of cells that are either naked or covered by scales, lorica or a cell wall. They can be single-celled, colonial, coenocytic or multicellular. Some Phaeophyceae (brown algae, seaweeds) develop as large multicellular thalli with differentiated tissues. All ochrophytes uniformly have tubular mitochondrial cristae. This is a common trait shared with their relatives, heterotrophic stramenopiles, as well as other closely related groups such as Rhizaria, Telonemia and Alveolata. As primarily photosynthetic eukaryotes, they are considered algae, distinguished from other groups of algae by specific morphological and ultrastructural traits, such as their flagella, chloroplasts and pigments.

As stramenopiles (=heterokonts), their swimming cells frequently display two markedly unequal flagella: an anterior flagellum ("tinsel") with straw-like hollow tripartite hairs called mastigonemes, and an immature posterior smooth flagellum ("whiplash") lacking these hairs. The ciliary transition zone of the flagellum generally has a transitional helix.

The ochrophytes are mostly photosynthetic. As such, they may possess one or more photosynthetic plastids (chloroplasts) per cell. Some groups contain species with leucoplasts, chloroplasts that have lost photosynthetic capacity and pigments but presumably continue to play a role in the synthesis of amino acids, lipids and heme groups. Ochrophytes have a distinct plastid ultrastructure in comparison to other algal groups. Their chloroplasts originate from an event of secondary endosymbiosis from a red alga, which lead to four surrounding membranes: two inner membranes, corresponding to the primary plastid membranes; a third membrane, corresponding to the plasma membrane of the red alga; and an outermost layer, corresponding to the phagosome membrane. This characteristic differentiates them from archaeplastid algae (glaucophytes, red algae and green algae), whose chloroplasts have only two membranes. The two outer layers of ochrophyte plastids are continguous with the endoplasmic reticulum (ER), together composing the chloroplast endoplasmic reticulum (CER), also known as the periplastidial endoplasmic reticulum (PER), which is often connected to the nuclear envelope. The tripartite flagellar hairs, characteristic of stramenopiles, are produced within either the PER or the nuclear envelope.

The periplastid compartment (PC), between the second and third layers, is a separate region that in other algal groups (i.e. cryptomonads and chlorarachniophytes) contains a nucleomorph, the vestigial nucleus of the secondary endosymbiont; however, no nucleomorphs are known within the ochrophytes. Instead, other structures have been observed within the PC, similarly to those seen in haptophytes and chromerid algae: "blob-like structures" where PC proteins are localized, and a vesicular network. Within the CER, there is a prominent region of tight direct contacts between the periplastid membrane and the inner nuclear envelope, where lipid transfers might occur, and perhaps exchange of other molecules.

Commonly, within the plastid stroma, three stacked thylakoids differentiate into the "girdle lamella", which runs around the periphery of the plastid, beneath the innermost membrane. The remaining thylakoids are arranged in stacks of three. In synchromophytes and aurearenophytes, a consortium of several plastids, each surrounded by two or three inner membranes respectively, is enveloped by a shared outer membrane.

Ochrophyte chloroplasts contain chlorophylls a and c as photosynthetic pigments, in addition to fucoxanthin. Chlorophyll a binds to thylakoids, while the c pigment is present in the stroma. The most frequent accessory pigment in ochrophytes is the yellow β-carotene. The golden-brown or brown pigmentation in diatoms, brown algae, golden algae and others is conferred by the xanthophyll fucoxanthin. In the yellow-green or yellow-brown raphidophyceans, eustigmatophyceans and xanthophyceans, vaucheriaxanthin is dominant instead. These pigment combinations extend their photosynthetic ability beyond chlorophyll a alone. Additionally, xanthophylls protect the photosystems from high intensity light.

Ochrophyte algae accumulate chrysolaminarin, a carbohydrate consisting of short chains of β-1,3-linked glucose molecules, as a storage product. It is stored in vesicles located within the cytoplasm, outside plastids, unlike other algae. Cytoplasmic lipid droplets are also common. They lack starch, which is the common storage product in green algae and plants.