The Cooper Union speech or address, known at the time as the Cooper Institute speech, was delivered by Abraham Lincoln on February 27, 1860, at Cooper Union, in New York City. Lincoln was not yet the Republican nominee for the presidency, as the convention was scheduled for May. It is considered one of his most important speeches. Some historians have argued that the speech was responsible for his victory in the presidential election later that year.

In the speech, Lincoln elaborated his views on slavery by affirming that he did not wish it to be expanded into the western territories and claiming that the Founding Fathers would agree with this position. The journalist Robert J. McNamara wrote, "Lincoln's Cooper Union speech was one of his longest, at more than 7,000 words. And it is not one of his speeches with passages that are often quoted. Yet, due to the careful research and Lincoln's forceful argument, it was stunningly effective."

Horace Greeley's New-York Tribune hailed it as "one of the most happiest and most convincing political arguments ever made in this City.... No man ever made such an impression on his first appeal to a New-York audience."

As 1860 dawned, Lincoln's political tides were turning. Although he had lost a chance at a Senate seat in the 1858 Illinois Senate elections, he now eyed the presidency. However, it was expected that "the office should seek the man", and Lincoln refrained from announcing his candidacy. In February 1860, he was invited to speak at Henry Ward Beecher's Plymouth Church in Brooklyn, New York, an invitation he eagerly accepted. Having not spoken in the East before, Lincoln was eager to make a good impression. He had a new suit fitted (at the cost of $100 (about $3,000 in current dollars)) and went to great pains to write a sophisticated and well-researched speech. His new suit was of little impact, as the suit still fit the massive and lanky Lincoln poorly. But his speech "was a masterful exploration of the political paths open to the nation".

By the time Lincoln got to New York, he had learned that the speech would instead be sponsored by the Young Men's Central Republican Union and would now be given at the eponymous Cooper Union. Lincoln hurriedly rewrote his speech for a less religious audience. The night before the speech, he painstakingly "review[ed] and typesett[ed]" it with the aid of journalist acquaintances.

The new audience proved to be very useful for Lincoln, as it now included Horace Greeley, who had the power to act as a presidential king-maker and was on a campaign to prevent the presidential nomination of his longtime friend, and now sworn rival, William H. Seward.

Lincoln was the third speaker in a series, going after Frank Blair (who would later serve as an advisor to Lincoln) and abolitionist Cassius M. Clay. New York Evening Post editor William Cullen Bryant provided a warm introduction. Lincoln's ungainly appearance, ill-fitting suit, and shrill voice gave an initially poor impression to listeners, but he soon warmed up, and his oratory improved. The clarity and logic of his speech quickly wiped away any doubts the audience had.

Lincoln's speech has three major parts, each building towards his conclusion. The first part concerns the founders and the legal positions they supported on the question of slavery in the territories. The second part is addressed to the voters of the Southern states by clarifying the issues between Republicans and Democrats. He rebukes claims made by the Democrats that they are "conservative", arguing instead that the Republicans' position on slavery is in fact the "conservative" policy, as Lincoln claims it coincides with the views of the American founding fathers, who he said opposed slavery. By supporting slavery, Lincoln claims that the Democrats are in opposition to the teachings of the founding fathers and "reject, and scout, and spit upon that old policy, and insist upon substituting something new." The final section is addressed to Republicans.