A French ensign is the flag flown at sea to identify a vessel as French. Several such ensigns have existed over the years as well as terrestrial flags based on the ensign motif.

The current French ensign is not, as the casual observer would think, identical to the French flag. Though both are blue, white and red, the French civil ensign has those colours in the proportion blue 30, white 33, and red 37. The intention is in fact to create a flag which, when seen moving at some distance, will appear to have columns of equal width; in addition, the slightly wider red column is intended to improve the flag's visibility at sea.

The current French ensign, with proportions different from those of the French flag.

As with the ensigns of other countries, the French ensign in the beginning of the 14th century was a banner of the royal arms, blue filled with golden French lilies. Sometimes it bears a white cross.

In 1365, Charles V changed to a blue flag with just three golden French lilies. However, reports as late as 1514 still occasionally mention the use of the lilies and cross flag.