The Santa Ynez Mountains are a portion of the Transverse Ranges, part of the Pacific Coast Ranges of the west coast of North America, and are one of the northernmost mountain ranges in Southern California. They are principally in Santa Barbara County, with an eastward extension into Ventura County, and are unusual in being an entirely east-west trending mountain range—one of the few in the United States.

The mountains extend from an eastern terminus at the canyon of the Ventura River and Matilija Creek, north of Ojai, west across the Santa Barbara County line, to immediately north of the city of Santa Barbara, and then west, paralleling the coast, to the city of Lompoc and Vandenberg Air Force Base. Before reaching Lompoc they diverge into two low ranges, separated by Jalama Creek, and then vanish into the Pacific Ocean. The mountains parallel the Channel Islands to the south, another east-west trending range, a geologic extension of the Santa Monica Mountains; the two ranges are about thirty miles apart. Principal summits in the Santa Ynez range include Divide Peak, 4707' (1434 m), La Cumbre Peak, 3985' (1214 m) (above Santa Barbara), and Santa Ynez Peak, 4298' (1310 m). There are several important passes, including Gaviota Pass in the western portion of the range, through which runs U.S. Highway 101, and San Marcos Pass near Santa Barbara, through which runs California Route 154.

Geologically the mountains are young, and almost entirely sedimentary. They were uplifted in the late Miocene epoch, about five million years ago, along the Santa Ynez Fault , a feature which is no longer considered to be active. Being young, the slopes are steep and the topography is extremely rugged; the gradient is highest on the northern side, above the trace of the fault. Dramatic sandstone formations are visible at many locations in the range. The predominant ground cover is chaparral, with coastal sage scrub , oak woodland and grasslands at lower elevations. Isolated stands of conifers and other evergreen trees such as bay laurel, tanbark oak and madrone grow on the higher parts of the northern slopes, which are cooler and have a somewhat higher annual rainfall than the southern slopes. At lower elevations on the southern side of the range, a virtually frost-free zone, avocados are grown in considerable quantity.

The climate of the mountain range is Mediterranean. Summers are warm and almost entirely rainless, save for occasional monsoonal showers in August and September, though in most years there is no rain between May and October. Winters are mild and can be rainy, with sometimes over 40 inches of rain falling in the mountains (while only 15 inches falls in the towns below due to the orographic effect of clouds dropping most of their moisture on the higher terrain). About one year in three snow falls on the higher peaks in the mountains, but it rarely stays for longer than a few days. In the late spring and early summer the mountains and areas below to the south are subject to occasional intense sundowner winds , a type of foehn wind in which the air is heated adiabatically and dried as it moves downslope. In dry years these winds pose an extreme fire hazard.

Large portions of the mountain range are in the Los Padres National Forest , although there are private inholdings, including some substantial communities, such as Painted Cave (above Santa Barbara). Famous residents have included Ronald Reagan, whose Rancho Cielo was at the top of the range west of Santa Barbara, and Jane Fonda, who also owned a ridgetop ranch into the late 1990s. No visits between the two are known to have occurred.

Occasionally extremely destructive fires originate in the Santa Ynez Mountains. In June 1990, a fire which began near Painted Cave burned south into the city of Santa Barbara, driven by sundowner winds, and destroyed over 500 homes.