Rossmoor is a planned census-designated place located in Orange County, California. As of the 2020 census, the CDP had a total population of 10,625 up from the 2010 census population of 10,244. The gated Leisure World retirement community in the city of Seal Beach is to the south of Rossmoor, Los Alamitos is to the east and north, and Long Beach is to the west (on the other side of the San Gabriel River, the 605 freeway and the border with Los Angeles County). The community of Rossmoor has two shopping centers within its boundaries, but only one—the Rossmoor Village Square, is now within the political boundaries of Rossmoor. A larger shopping center, the Rossmoor Business Center, was annexed, despite many protests, by the City of Seal Beach in 1967. The Center has been remodeled several times and was renamed the Shops at Rossmoor in the early 2000s.

The Rossmoor community was developed from 1955 through 1961 by Ross W. Cortese, who had earlier developed the architecturally-significant Lakewood Rancho Estates in Long Beach (1953) and the Frematic Homes (1954) in Anaheim, just north and west of Disneyland. Cortese filed his first subdivision map for Rossmoor on October 20, 1956, and filed his thirteenth and final map on March 21, 1960. Cortese's original partners in securing the first large parcels of land that would become Rossmoor included California governor Goodwin Knight and Judge Alfred Gittelson, who had also partnered with him in the Lakewood Rancho Estates and the Frematic homes. (It was Gittelson who later agreed to annex his Rossmoor Shopping Center property to Seal Beach in the late 1960s.) After Rossmoor, Cortese would construct the first of his very successful Leisure World gated retirement communities in that part of Seal Beach which is immediately south of Rossmoor. The Rossmoor community is easy to recognize because of its red brick "signature wall" that borders the entire unincorporated community. It is a signature of the Rossmoor community that differentiates it from neighboring cities Los Alamitos and Seal Beach.

There are 3,430 single family homes, 1 apartment complex (Accent Apartment Homes, originally Rossmoor Manor on Wallingsford Road), and 1 town house complex (Rossmoor Town Houses with addresses at 12100 Montecito Road or 3342 Bradbury Road) within Rossmoor.

According to a June 2006 consultant's report to the Rossmoor Planning Committee, the most cost effective solution for Rossmoor would be to be annexed by the city of Seal Beach.

Although it is not part of the city of Los Alamitos, it is served under the Los Alamitos "sphere of influence" and was proposed to be annexed to the city by 2007 after the Local Agency Formation Commission reviewed the proposal.

Another proposal would combine Rossmoor with adjacent Seal Beach and Los Alamitos to form one city.

Rossmoor is served by the Rossmoor/Los Alamitos Area Sewer District. The mainly residential area is also served by the Rossmoor Community Services District (CSD), which serves the community as an independent functionary does. The CSD provides services related to public recreation facilities and park development for Rush Park, Rossmoor Park, two mini parks and the Montecito Center. The CSD also administers street lighting, road rights-of-way services, and manages the street sweeping contract. However, the Rossmoor CSD cannot make decisions about the future of the community, which is all delegated to the County of Orange's Unincorporated Islands Program and the Orange County Board of Supervisors.

At the May 22, 2008 meeting, OC LAFCO recommended that the Rossmoor Incorporation Plan go to a vote of the community. A vote by residents on the issue of incorporation was held on November 4, 2008. The proposition for cityhood was resoundingly defeated by a vote of 28% to 72% indicating that the vast majority of residents desired not to be incorporated as a city. Residents also voted down three separate utility tax proposals by a wide margin.