The Second Continental Congress adopted the Declaration of Independence in Philadelphia. Its delegates met in the Pennsylvania State House, now known as Independence Hall, where Congress approved the document on July 4, 1776.
The Second Continental Congress first convened in Philadelphia on May 10, 1775, after the Revolutionary War had begun. It acted as the colonies’ de facto national government, directing the war effort, appointing diplomats, issuing currency, and organizing military supplies. In 1776, it handled both the Lee Resolution declaring independence and the Declaration that explained that decision.
Philadelphia is the canonical answer for the adoption site, although the Congress later moved temporarily to other cities during the war, including Baltimore, Lancaster, and York. This sometimes causes confusion in questions about where the Continental Congress met generally. For the Declaration’s adoption, however, the relevant meeting place was the Pennsylvania State House in Philadelphia.