John Adams (October 30, 1735 – July 4, 1826) was an American statesman, attorney, diplomat, writer, and Founding Father who served as the second president of the United States from 1797 to 1801. Before his presidency, he was a leader of the American Revolution that achieved independence from Great Britain. During the latter part of the war and in the early years of the nation, he served as a diplomat in Europe. He was the first person to hold the office of vice president of the United States, serving from 1789 to 1797. Adams was a dedicated diarist and regularly corresponded with many important contemporaries, including his wife and adviser Abigail Adams as well as his friend and rival Thomas Jefferson.

A lawyer and political activist prior to the Revolution, Adams was devoted to the right to counsel and presumption of innocence. He defied anti-British sentiment and successfully defended British soldiers against murder charges arising from the Boston Massacre. Adams was a Massachusetts delegate to the Continental Congress and became a leader of the revolution. He assisted Jefferson in drafting the Declaration of Independence in 1776. As a diplomat in Europe, he helped negotiate a peace treaty with Great Britain and secured vital governmental loans. Adams was the primary author of the Massachusetts Constitution in 1780, which influenced the United States constitution, as did his essay Thoughts on Government.

Adams was elected to two terms as vice president under President George Washington and was elected as the United States' second president in 1796. He was the only president elected under the banner of the Federalist Party. During his single term, Adams encountered fierce criticism from the Jeffersonian Republicans and from some in his own Federalist Party, led by his rival Alexander Hamilton. Adams signed the controversial Alien and Sedition Acts, and built up the Army and Navy in the undeclared naval war (called the "Quasi-War") with France. During his term, he became the first president to reside in the executive mansion now known as the White House.

In his bid in 1800 for reelection to the presidency, opposition from Federalists and accusations of despotism from Jeffersonians led to Adams losing to his vice president and former friend Jefferson, and he retired to Massachusetts. He eventually resumed his friendship with Jefferson by initiating a correspondence that lasted fourteen years. He and his wife generated the Adams political family, a line of politicians, diplomats, and historians. It includes their son John Quincy Adams, the sixth president. John Adams died on July 4, 1826 – the fiftieth anniversary of the adoption of the Declaration of Independence – hours after Jefferson's death. Adams and his son are the only presidents of the first twelve who never owned slaves. Surveys of historians and scholars have favorably ranked his administration.