The Peninsular Ranges are a group of mountain ranges which stretch 900 miles from southern California to the southern tip of Baja California, part of the North American Coast Ranges that run along the Pacific coast from Alaska to Mexico. The Peninsular ranges run predominantly north-south, unlike the Transverse Ranges to their north, which mostly run east-west.

The Peninsular Ranges include the Santa Ana Mountains, San Jacinto Mountains and Laguna Mountains of southern California with the Sierra Juarez, San Pedro Martir, and La Giganta mountains of Baja California. Mount Palomar, home to Palomar Observatory, is in the Peninsular Ranges in San Diego County.

Several terrestrial ecoregions cover portions of the Peninsular Ranges. Most of the ranges are in the Nearctic ecozone. On the coast side of the ranges, the California chaparral and woodlands ecoregion covers the northern portion of the range, in southern California and northern Baja California. The Baja California desert ecoregion runs along the coast side of the southern portion of the Peninsular Ranges. On the eastern side of the ranges, the Sonoran Desert ecoregion covers southeastern California and northeastern Baja California as far south as the town of Loreto, Mexico. Anza-Borrego Desert State Park, on the eastern side of the Peninsular Ranges in San Diego County, is famous for its springtime profusion of Sonoran Desert wildflowers. The Gulf of California xeric scrub ecoregion covering the southeastern portion of the peninsula and the islands of the Gulf of California.

The higher portions of the Peninsular Ranges, especially the west-facing slopes, are home to coniferous and mixed forests. Cleveland National Forest covers much of the southern California Peninsular Ranges, and includes oak woodlands and forests of Jeffrey Pine (Pinus jeffreyi) and Coulter pine (P. coulteri). The Sierra Juarez and San Pedro Martir pine-oak forests cover the upper slopes of the Sierra Juarez and San Pedro Martir ranges. These isolated forests, predominantly pines, white fir, juniper, and oaks, harbor many rare and endemic species.

The southern end of the Baja Peninsula, including the Sierra de la Laguna, was formerly an island, and evolved in relative isolation from the northern part of the peninsula. The flora and fauna share many affinities with Central America, and are considered part of the Neotropic ecozone. It includes three distinct ecoregions, the Sierra de la Laguna dry forests, Sierra de la Laguna pine-oak forests, and San Lucan xeric scrub.