The National American Woman Suffrage Association (NAWSA) was an organization formed on February 18, 1890, to advocate in favor of women's suffrage in the United States. It was created by the merger of two existing organizations, the National Woman Suffrage Association (NWSA) and the American Woman Suffrage Association (AWSA). Its membership, which was about seven thousand at the time it was formed, eventually increased to two million, making it the largest voluntary organization in the nation. It played a pivotal role in the passing of the Nineteenth Amendment to the United States Constitution, which in 1920 guaranteed women's right to vote.

Susan B. Anthony, a long-time leader in the suffrage movement, was the dominant figure in the newly formed NAWSA. Carrie Chapman Catt, who became president after Anthony retired in 1900, implemented a strategy of recruiting wealthy members of the rapidly growing women's club movement, whose time, money and experience could help build the suffrage movement. Anna Howard Shaw's term in office, which began in 1904, saw strong growth in the organization's membership and public approval.

After the Senate decisively rejected the proposed women's suffrage amendment to the U.S. Constitution in 1887, the suffrage movement had concentrated most of its efforts on state suffrage campaigns. In 1910 Alice Paul joined the NAWSA and played a major role in reviving interest in the national amendment. After continuing conflicts with the NAWSA leadership over tactics, Paul created a rival organization, the National Woman's Party.

When Catt again became president in 1915, the NAWSA adopted her plan to centralize the organization, and work toward the suffrage amendment as its primary goal. This was done despite opposition from Southern members who believed that a federal amendment would erode states' rights. With its large membership and the increasing number of women voters in states where suffrage had already been achieved, the NAWSA began to operate more as a political pressure group than an educational group. It won additional sympathy for the suffrage cause by actively cooperating with the war effort during World War I. On February 14, 1920, several months prior to the ratification of the Nineteenth Amendment, the NAWSA transformed itself into the League of Women Voters, which is still active.

The demand for women's suffrage in the United States was controversial even among women's rights activists in the early days of the movement. In 1848, a resolution in favor of women's right to vote was approved only after vigorous debate at the Seneca Falls Convention, the first women's rights convention. By the time of the National Women's Rights Conventions in the 1850s, the situation had changed, and women's suffrage had become a preeminent goal of the movement. Three leaders of the women's movement during this period, Lucy Stone, Elizabeth Cady Stanton, and Susan B. Anthony, played prominent roles in the creation of the NAWSA many years later.

In 1866, just after the American Civil War, the Eleventh National Women's Rights Convention transformed itself into the American Equal Rights Association (AERA), which worked for equal rights for both African Americans and white women, especially suffrage. The AERA essentially collapsed in 1869, partly because of disagreement over the proposed Fifteenth Amendment to the United States Constitution, which would enfranchise African American men. Leaders of the women's movement were dismayed that it would not also enfranchise women. Stanton and Anthony opposed its ratification unless it was accompanied by another amendment that would enfranchise women. Stone supported the amendment. She believed that its ratification would spur politicians to support a similar amendment for women. She said that even though the right to vote was more important for women than for black men, "I will be thankful in my soul if any body can get out of the terrible pit."

In May 1869, two days after the acrimonious debates at what turned out to be the final AERA annual meeting, Anthony, Stanton and their allies formed the National Woman Suffrage Association (NWSA). In November 1869, the American Woman Suffrage Association (AWSA) was formed by Lucy Stone, her husband Henry Blackwell, Julia Ward Howe and their allies, many of whom had helped to create the New England Woman Suffrage Association a year earlier as part of the developing split. The bitter rivalry between the two organizations created a partisan atmosphere that endured for decades.

Even after the Fifteenth Amendment was ratified in 1870, differences between the two organizations remained. The AWSA worked almost exclusively for women's suffrage while the NWSA initially worked on a wide range of issues, including divorce reform and equal pay for women. The AWSA included both men and women among its leadership while the NWSA was led by women. The AWSA worked for suffrage mostly at the state level while the NWSA worked more at the national level. The AWSA cultivated an image of respectability while the NWSA sometimes used confrontational tactics. Anthony, for example, interrupted the official ceremonies at the 100th anniversary of the Declaration of Independence to present NWSA's Declaration of Rights for Women. Anthony was arrested in 1872 for voting, which was still illegal for women, and was found guilty in a highly publicized trial.