Erythranthe, the monkey-flowers and musk-flowers, is a diverse plant genus with more than 120 members (as of 2022) in the family Phrymaceae. Erythranthe was originally described as a separate genus, then generally regarded as a section within the genus Mimulus, and recently returned to generic rank. Mimulus sect. Diplacus was segregated from Mimulus as a separate genus at the same time. Mimulus remains as a small genus of eastern North America and the Southern Hemisphere. Molecular data show Erythranthe and Diplacus to be distinct evolutionary lines that are distinct from Mimulus as strictly defined, although this nomenclature is controversial.

Member species are usually annuals or herbaceous perennials. Flowers are red, pink, or yellow, often in various combinations. A large number of the Erythranthe species grow in moist to wet soils with some growing even in shallow water. They are not very drought resistant, but many of the species now classified as Diplacus are. Species are found at elevations from oceanside to high mountains as well as a wide variety of climates, though most prefer wet areas such as riverbanks.

The largest concentration of species is in western North America, but species are found elsewhere in the United States and Canada, as well as from Mexico to Chile and eastern Asia. Pollination is mostly by either bees or hummingbirds. Member species are widely cultivated and are subject to several pests and diseases. Several species are listed as threatened by the International Union for Conservation of Nature.

Erythranthe is a highly diverse genus with the characteristics unifying the various species being axile placentation and long pedicels. Other characteristics of species can vary widely, especially between the sections, and even within some sections. Some species of Erythranthe are annuals and some are perennials. Flowers are red, pink, purple, or yellow, often in various combinations and shades of those colors. Some species produce copious amounts of aromatic compounds, giving them a musky odor (hence "musk-flowers"). Erythranthe is used as food by the larvae of some Lepidoptera species, such as the mouse moth (Amphipyra tragopoginis), as a main part of their diet.

Within the section Erythranthe, stems and leaves range from glabrous to hirsute, and are generally glandular. Leaves can be oblong, elliptical, or oval, with small tooths. Fruiting pedicels are longer than calyces. Calyces have sharp, definite angles and flat sides. Corollas are deciduous, relatively large (tube-throat 8–42 mm (0.3–1.7 in) long), and strongly red to purplish, magentarose, pink, or white, rarely yellow.

Erythranthe guttata is the most widespread of the genus Erythranthe and its characteristics are fairly representative of the genus. E. guttata is 5–80 cm (2–31 in) tall with disproportionately large 5–20 mm (0.2–0.8 in) long, tubular flowers. Leaves are opposite and oval, 1–10 cm (0.4–3.9 in) long. The species as strictly defined is perennial and spreads with stolons or rhizomes. The stem may be erect or recumbent. In the latter form, roots may develop at lower leaf nodes. Sometimes dwarfed, it may be hairless or have some hairs. Leaves are opposite, round to oval, usually coarsely and irregularly toothed or lobed. The bright yellow flowers are born on a raceme, most often with five or more flowers. The calyx has five lobes that are much shorter than the flower. Each flower has bilateral symmetry and has two lips. The upper lip usually has two lobes; the lower, three. The lower lip may have one large to many small red to reddish brown spots. The opening to the flower is hairy. The fruit is a two-valved capsule 1 cm (0.4 in) long, containing many seeds.

Erythranthe alsinoides is similar to several species found in the Pacific Northwest. It is an annual herb that blooms from April–June with a preference for shady and moist dense habitats. The plant is hairy to slightly hairy and grows from 0.5–3 dm (2–12 in) tall. The stems are often reddish. The leaves are opposite and have a few prominent upper veins. Blades are 0.5–2.5 cm (0.2–1.0 in) long. The petiole is about the same length. The flowers are yellow with reddish-brown spots, usually on the lower lip, and the upper and lower lips have fused, growing 8–14 cm (3.1–5.5 in). Each flower is attached by a pedicel. The fruits are capsules.

The derivation of Erythranthe is from Greek ἐρυθρός ("erythros"), red, with ἄνθος ("anthos"), flower. They are called monkey-flowers because some species have flowers shaped like a monkey's face. The widely used generic name, Latin mimus meaning "mimic actor", from the Greek mimos meaning "imitator" also alludes to the fancied monkey resemblance. The stem of Erythranthe can be either smooth or hairy, and this is known in a few species to be a trait determined by a simple allelic difference. At least E. lewisii is known to possess "flypaper-type" traps and is apparently protocarnivorous, supplementing its nutrients with small insects. Variations in color largely reflect concentrations of anthocyanins. The species that are subshrubs with woody stems were originally placed in the section Diplacus, and this was subsequently made a separate genus. Diplacus is clearly derived from within Mimulus, broadly defined, and was not usually considered to be a separate genus.