Orange County is a county in Southern California, United States. Its county seat is Santa Ana. According to the 2000 Census, its population was 2,846,289, making it the second most populous county in the state of California, and the fifth most populous in the United States. The state of California estimates its population as of 2007 to be 3,098,121 people, dropping its rank to third, behind San Diego County by 148 people.

Unlike other population centers, Orange County residents take their county name as their label of cultural identity, whereas most population centers in the United States tend to be identified by a major city. There is no defined urban center to Orange County as there generally is in other areas with one dominant municipal entity. Orange County is almost uniformly sub-urban with a few interspersed vestigial patches of agriculture. Five Orange County cities have populations exceeding 170,000 while none have populations surpassing 360,000.

It is also famous as a tourist destination, as the county is home to such attractions as Disneyland and Knott's Berry Farm, as well as sandy beaches for swimming and surfing, yacht harbors for sailing and pleasure boating, and extensive area devoted to parks and open space for golf, tennis, hiking, kayaking, cycling, skateboarding, and other . It is often portrayed in the media as a quite affluent and politically conservative region. It is at the center of Southern California's Tech Coast, with Irvine being the primary business hub.

Thirty-four incorporated cities are located in Orange County; the newest is Aliso Viejo and Anaheim being the oldest. Seven of these cities are among the 200 largest cities in the United States, Santa Ana being the largest in the county.