The northern cardinal (Cardinalis cardinalis), also commonly known as the common cardinal, red cardinal, or simply cardinal, is a bird in the genus Cardinalis. It can be found in southeastern Canada, through the eastern United States from Maine to Minnesota to Texas, New Mexico, southern Arizona, southern California and south through Mexico, Belize, and Guatemala. It is also an introduced species in a few locations such as Bermuda and all major islands of Hawaii since its introduction in 1929. Its habitat includes woodlands, gardens, shrublands, and wetlands. It is the state bird of Illinois, Indiana, Kentucky, North Carolina, Ohio, Virginia, and West Virginia.

The northern cardinal is a mid-sized perching songbird with a body length of 21–23 cm (8.3–9.1 in) and a crest on the top of its head. The species expresses sexual dimorphism: Females are a reddish olive color, and have a gray mask around the beak, while males are a vibrant red color, and have a black mask on the face, as well as a larger crest. Juvenile cardinals do not have the distinctive red-orange beak seen in adult birds until they are almost fully mature. On hatching, their beaks are grayish-black and they do not become the trademark orange-red color until they acquire their final adult plumage in the fall.

The northern cardinal is mainly granivorous but also feeds on insects and fruit. The male behaves territorially, marking out his territory with song. During courtship, the male feeds seed to the female beak-to-beak. The northern cardinal's clutch typically contains three to four eggs, with two to four clutches produced each year. It was once prized as a pet, but its sale was banned in the United States by the Migratory Bird Treaty Act of 1918.

The northern cardinal was described by Carl Linnaeus in the 1758 10th edition of Systema Naturae. It was originally categorized as Loxia cardinalis, a genus which now contains only crossbills. In 1838, it was recategorized as Cardinalis virginianus. In 1918, the scientific name was sometimes replaced with Richmondena cardinalis. In 1983, the scientific name was changed again to Cardinalis cardinalis and the common name was changed to "northern cardinal" to avoid confusion with the several other species also termed cardinals.

The cardinal is named after cardinals of the Roman Catholic Church, who wear distinctive red robes and caps. The term "northern" in the common name refers to its range, as it is the northernmost cardinal species known.

Northern cardinals are numerous across the eastern United States from the southern half of Maine to Minnesota to the Texas-Mexico border and in Canada in the southern portions of Ontario, Quebec, New Brunswick and Nova Scotia, all the way east to Cape Breton Island. Its range also extends south through Mexico to the Isthmus of Tehuantepec, northern Guatemala, and northern Belize. An allopatric population is found on the Pacific slope of Mexico from Jalisco to Oaxaca (this population is not shown on the range map). The species was introduced to Bermuda in 1700. It has also been introduced in Hawaii. Its natural habitat is in woodlands, gardens, shrublands, and wetlands.

In 1929, Forbush described the species as rare, and by 1955 Griscom reported the bird to be "pushing northward" when recorded annually at feeding stations. Audubon data shows that the population has grown rapidly in Massachusetts since 1960. In Massachusetts, the species is most abundant in the east, especially in areas where dense cover is interspersed with open areas, such as woodland edges, brushy fields, wooded wetlands, parks, and suburban areas. They tend to avoid extensive woodlands.

The northern cardinal is a mid-sized songbird with a body length of 21–23.5 cm (8.3–9.3 in) and a wingspan of 25–31 cm (9.8–12.2 in). The adult weighs from 33.6–65 g (1.19–2.29 oz), with an average 44.8 g (1.58 oz). The male averages slightly larger than the female.