Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is fully regenerated in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness.

Rhodopsin was discovered by Franz Christian Boll in 1876. The name rhodopsin derives from Ancient Greek ῥόδον (rhódon) for "rose", due to its pinkish color, and ὄψις (ópsis) for "sight". It was coined in 1878 by the German physiologist Wilhelm Friedrich Kühne (1837–1900).

When George Wald discovered that rhodopsin is a holoprotein, consisting of retinal and an apoprotein, he called it opsin, which today would be described more narrowly as apo-rhodopsin. Today, the term opsin refers more broadly to the class of G-protein-coupled receptors that bind retinal and as a result become a light-sensitive photoreceptor, including all closely related proteins. When Wald and colleagues later isolated iodopsin from chicken retinas, thereby discovering the first known cone opsin, they called apo-iodopsin photopsin (for its relation to photopic vision) and apo-rhodopsin scotopsin (for its use in scotopic vision).

Rhodopsin is a protein found in the outer segment discs of rod cells. It mediates scotopic vision, which is monochromatic vision in dim light. Rhodopsin most strongly absorbs green-blue light (~500 nm) and appears therefore reddish-purple, hence the archaic term "visual purple".

Several closely related opsins differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have, including rhodopsin, nine opsins, as well as cryptochrome (light-sensitive, but not an opsin).

Rhodopsin, like other opsins, is a G-protein-coupled receptor (GPCR). GPCRs are chemoreceptors that embed in the lipid bilayer of the cell membranes and have seven transmembrane domains forming a binding pocket for a ligand. The ligand for rhodopsin is the vitamin A-based chromophore 11-cis-retinal, which lies horizontally to the cell membrane and is covalently bound to a lysine residue (lys296) in the seventh transmembrane domain through a Schiff-base. However, 11-cis-retinal only blocks the binding pocket and does not activate rhodopsin. It is only activated when 11-cis-retinal absorbs a photon of light and isomerizes to all-trans-retinal, the receptor activating form, causing a configurational change in the rhodopsin, which activate a phototransduction cascade. Thus, a chemoreceptor is converted to a light or photo(n)receptor.

The retinal binding lysine is conserved in almost all opsins, only a few opsins having lost it during evolution. Opsins without the lysine are not light sensitive, including rhodopsin. Rhodopsin is made constitutively (continuously) active by some of those mutations even without light. Also wild-type rhodopsin is constitutively active, if no 11-cis-retinal is bound, but much less. Therefore 11-cis-retinal is an inverse agonist. Such mutations are one cause of autosomal dominant retinitis pigmentosa. Artificially, the retinal binding lysine can be shifted to other positions, even into other transmembrane domains, without changing the activity.

The rhodopsin of cattle has 348 amino acids, the retinal binding lysine being Lys296. It was the first opsin whose amino acid sequence and 3D-structure were determined. Its structure has been studied in detail by x-ray crystallography on rhodopsin crystals. Several models (e.g., the bicycle-pedal mechanism, hula-twist mechanism) attempt to explain how the retinal group can change its conformation without clashing with the enveloping rhodopsin protein pocket. Recent data support that rhodopsin is a functional monomer, instead of a dimer, which was the paradigm of G-protein-coupled receptors for many years.