The Continental Union Flag (often referred to as the first American flag, Cambridge Flag, and Grand Union Flag) was the flag of the United Colonies from 1775 to 1776, and the de facto flag of the United States until 1777, when the 13 star flag was adopted by the Continental Congress. It was a variant of the British 'Red Ensign.'

The Continental Union Flag was so called because it combined the British Union flag (denoting the kingdoms of England and Scotland) with thirteen stripes (representing the United Colonies). The canton consists of the Union flag, while the field is thirteen horizontal stripes, alternating red and white. The flag made its first appearance on December 3, 1775, when it was hoisted at the commissioning of Admiral Esek Hopkins' flagship on the western shore of the Delaware River at Philadelphia.

Known during the American Revolution as the Continental Union Flag, Continental Colours, American flag, and flag of the United Colonies; the name was derived from a combination of the words "Continental," referring to the Continental Congress, and "Union Flag," referring to the British Union Flag of 1707 then flown in America.

Although the flag is often referred to as the "Cambridge Flag" and "Grand Union Flag," the terms did not come into use until the 19th century. While it has been claimed the more recent moniker, Grand Union Flag, was first applied to the flag by G. Henry Preble in his Reconstruction era book Our Flag, the first substantiated use of the name came from Philadelphia resident T. Westcott in 1852 when replying to an inquiry made in Notes and Queries, a London periodical, as to the origin of the U.S. flag.

In a letter to members of the Continental Congress from Virginia dated January 5, 1776, the Naval Committee described the Continental Union Flag as the British "Union flag... striped red and white in the field." Having seen the Continental Union Flag flying aboard Admiral Esek Hopkins' flagship a few days later, a Philadelphia resident further defined the flag as a British "Union flag, with 13 stripes in the field, emblematical of the Thirteen United Colonies."

British author and professor and Nick Groom believes incorporation of the Union Flag of 1707 in the canton of the Continental Union Flag suggests the Americans adopted it, not as a protest against the British Ministry, but as a profession of loyalty to King George. This view is shared by Laurie Calkhoven, a biographer of George Washington, who suggests it was designed to reflect their hope for reconciliation.

The Continental Union Flag is strikingly similar to the Flag of the East India Company. The red and white stripes on that flag, however, varied from nine to fifteen. One theory is that Americans would have been somewhat familiar with the East India Company flag and it may have influenced the design.

By the end of 1775, during the first year of the Revolutionary War, the Continental Congress operated as a de facto war government, who had authorized the creation of the Continental Army, the Navy, and Marines. A new flag was needed to represent both the Congress and the United Colonies, with a banner distinct from the Red Ensign, White Ensign, and Blue Ensign flown by British ships and the Union Flag carried on land by British troops. The emerging states had been using their own independent flags, with Massachusetts Bay using the Taunton Flag, and New York using the George Rex Flag, prior to the adoption of united colors. It is not known for certain when or by whom the Continental Union Flag design was created, but it could easily be produced by sewing white stripes onto the Red Ensign.