The Santa Ynez River is one of the largest rivers on the Central Coast of California. It is 92 miles (148 km) long, flowing from east to west through the Santa Ynez Valley, reaching the Pacific Ocean at Surf, near Vandenberg Space Force Base and the city of Lompoc.

The river drains the north slope of the Santa Ynez Mountains, the south slope of the San Rafael Mountains, as well as much of the southern half of Santa Barbara County. Its drainage basin is 896 square miles (2,320 km²) in area. The river's flow is highly variable. It usually dries up almost completely in the summer, but can become a raging torrent in the winter. The river has three dams which can impound a total of 210,000 acre-feet (260,000,000 m³) of water in wet years.

The river was first named by the Spanish Portolà expedition, first European land exploration of Alta California, which camped near the river mouth on August 30, 1769. Apparently unable to agree on a single name, expedition diarists recorded three. Engineer Miguel Costansó wrote "Río Grande de San Verardo". Franciscan missionary Juan Crespi noted two additional names; "San Bernardo" and "Santa Rosa". None of the three names remain attached to any feature in the area.

Instead, the river and mountains took the name of Mission Santa Inés (also spelled "Ynés" or "Ynéz" in New Spain), which was established in 1804 (dedicated to Saint Agnes of Rome, virgin and martyr). According to the USGS, variant and historical names of the Santa Ynez River include La Purisima River, Rio De La Purisima, Rio De Calaguasa, Rio Santa Rosa, Rio De Santa Ines, and Rio De Santa Ynes.

The Santa Ynez River originates in Los Padres National Forest, on the northern slope of the Santa Ynez Mountains near Divide Peak and the Ventura County border. The river flows west, collecting various headwater tributaries. The Upper Santa Ynez Campground is located near the river's source. After flowing through Billiard Flats the river enters Jameson Lake, the reservoir impounded by Juncal Dam. Below the dam, Alder Creek joins the Santa Ynez River from the south. At times water from Alder Creek is diverted into Jameson Lake via a tunnel.

Continuing its generally westward course, the Santa Ynez flows by several campgrounds and canyons, including Blue Canyon. Mono Creek joins from the north just as the Santa Ynez flows into Gibraltar Reservoir, impounded by Gibraltar Dam. Below this dam the river passes several campgrounds as well as facilities such as the Los Prietos Ranger Station. Paradise Road runs along the river. Continuing west, the river passes Fremont Campground near the mouth of Red Rock Canyon.

West of Red Rock Canyon the river leaves Los Padres National Forest and its valley widens considerably. Kelly Creek joins from the south, draining Los Laureles Canyon and Cold Spring Canyon. State Route 154, which crosses the Santa Ynez Mountains via San Marcos Pass, enters the Santa Ynez River valley at this point and follows the river for several miles to the west. Hot Spring Canyon joins from the south just before the Santa Ynez River enters Lake Cachuma.

Lake Cachuma, the largest reservoir on the river, is approximately 5 miles (8.0 km) in length. Several tributaries join the Santa Ynez River in Lake Cachuma, including Santa Cruz Creek and Cachuma Creek from the north and a number of smaller streams from the south. The lake area is designated as the Lake Cachuma Recreation Area. Cachuma County Park, near Tequepis Point, provides lake access. Water from the lake is diverted into the Tecolote Tunnel, which passes south under the mountains to the Santa Barbara area.