Nisei (二世, "second generation") is a Japanese-language term used in countries in North America and South America to specify the ethnically Japanese children born in the new country to Japanese-born immigrants, or Issei. The Nisei, or second generation, in turn are the parents of the Sansei, or third generation. These Japanese-language terms derive from ichi, ni, san, "one, two, three", the ordinal numbers used with sei (see Japanese numerals). Though nisei means "second-generation immigrant", it more specifically often refers to the children of the initial diaspora, occurring during the period of the Empire of Japan in the late 19th and early 20th centuries and overlapping in the U.S. with the G.I. and silent generations.

Although the earliest organized group of Japanese emigrants left Japan centuries ago, and a later group settled in Mexico in 1897, today's largest populations of Japanese immigrants and their descendants are concentrated in four countries: Brazil (2 million), the United States (1.5 million), Canada (130,000), and Peru (100,000).

Some US Nisei were born after the end of World War II during the baby boom. Most Nisei, however, who were living in the western United States during World War II, were forcibly interned with their parents (Issei) after Executive Order 9066 was promulgated to exclude everyone of Japanese descent from the West Coast areas of California, Oregon, Washington, and Alaska. It has been argued that some Nisei feel caught in a dilemma between their Nisei parents and other Americans. The Nisei of Hawaii had a somewhat different experience.

In the United States, two representative Nisei were Daniel Inouye and Fred Korematsu. Hawaiian-born Daniel Ken Inouye (井上 建, Inoue Ken; 1924–2012) was one of many young Nisei men who volunteered to fight in the nation's military when restrictions against Japanese-American enlistment were removed in 1943. Inouye later went on to become a U.S. Senator from Hawaii after it achieved statehood.

Fred Toyosaburo Korematsu (是松 豊三郎, Korematsu Toyosaburō; 1919–2005) was one of many Japanese-American citizens living on the West Coast who resisted internment during World War II. In 1944, Korematsu lost a U.S. Supreme Court challenge to the wartime internment of Japanese Americans but gained vindication decades later. The Presidential Medal of Freedom, the highest civilian honor in the United States, was awarded to Korematsu in 1998. At the White House award ceremonies, President Bill Clinton explained, "In the long history of our country's constant search for justice, some names of ordinary citizens stand for millions of souls. Plessy, Brown, Parks ... to that distinguished list, today we add the name of Fred Korematsu."

The overwhelming majority of Japanese Americans had reacted to the internment by acquiescing to the government's order, hoping to prove their loyalty as Americans. To them, Korematsu's opposition was treacherous to both his country and his community. Across the span of decades, he was seen as a traitor, a test case, an embarrassment and, finally, a hero.

Brazil is home to the largest Japanese population outside of Japan, estimated to number more than 1.5 million (including those of mixed-race or mixed-ethnicity), more than that of the 1.2 million in the United States. The Nisei Japanese Brazilians are an important part of the ethnic minority in that South American nation.

Within Japanese-Canadian communities across Canada, three distinct subgroups developed, each with different sociocultural referents, generational identity, and wartime experiences.