In the context of the 20th-century history of the United States, the Second Great Migration was the migration of more than 5 million African Americans from the South to the Northeast, Midwest and West. It began in 1940, through World War II, and lasted until 1970. It was much larger and of a different character than the first Great Migration (1916–1940), where the migrants were mainly rural farmers from the South and only came to the Northeast and Midwest.

In the Second Great Migration, not only the Northeast and Midwest continued to be the destination of more than 5 million African Americans, but also the West as well, where cities like Los Angeles, Oakland, Phoenix, Portland, and Seattle offered skilled jobs in the defense industry. Most of these migrants were already urban laborers who came from the cities of the South. In addition, African Americans were still treated with discrimination in parts of the country, and many sought to escape this.

Before the arrival of Black Southerners, the Black community in Los Angeles and California was mainly rooted in a complex Black identity with Afro-Mexicans of mixed Spanish, indigenous and African ancestry. By 1821, Mexico had abolished slavery as part of the Atlantic slave trade, and thus, these Afro-Mexicans were allowed to assimilate earlier into a society that later had become the United States following the Mexican–American War.

Compared to the more rural migrants of the period 1910–1940, many African Americans in the South were already living in urban areas and had urban job skills before they relocated. They moved to take jobs in the burgeoning industrial cities in the North and West, including the defense industry during World War II. Workers who were limited to segregated, low-skilled jobs in some cities were able to get highly skilled, well-paid jobs at California, Oregon, and Washington shipyards.

By the end of the Second Great Migration, African Americans had become a highly urbanized population. More than 80% lived in cities, a greater proportion than among the rest of American society. 53% remained in the Southern United States, while 40% lived in the Northeast and North Central states and 7% in the West.

While African Americans were often relegated to support roles during World War II, often these roles could be exceedingly hazardous. An accidental munitions explosion at Port Chicago, California, claimed the lives of over 200 African American sailors in 1944. Some sailors refused to resume work until conditions were made less hazardous. Up to 50 were imprisoned after a court martial convicted them of mutiny.

Further up the West Coast, high-paying shipbuilding war jobs attracted large numbers of African-Americans into the small existing communities. Their numbers in Seattle, Washington, tripled; the numbers in Portland, Oregon, quadrupled. The newcomers became permanent residents, building up black political influence, strengthening civil rights organizations such as the NAACP, calling for antidiscrimination legislation. On the negative side, racial tensions increased, both black and white residential areas deteriorated from overcrowding, and inside the black community there were angry words between "old settlers" and recent arrivals for leadership in the black communities.

Like participants in the Great Migration, those in the Second Great Migration were motivated to move for economic reasons. The economic deprivation suffered by southern African Americans prior to the Great Migration has been well documented. Plantation agriculture such as sharecropping limited African Americans to gain and work up the "agricultural ladder". The agricultural ladder was a system in which a worker's status determined their position, not how skilled or valuable they were. Because of segregation, African American men were placed in agricultural jobs and women were placed in domestic services. These conditions had little to no change from the early decades of the twentieth century, which was a powerful incentive for African American southerners to leave and go look for opportunity elsewhere.^{[citation needed]}