Ellicott's Stone, Boundary marker in Mobile County, Alabama, United States.

Ellicott's Stone is a sandstone block standing about 2 feet (0.6 meters) tall with English inscriptions on its north face and Spanish inscriptions on its south face, marking the 31st parallel of latitude. The stone now sits in a historical park and shows the precise technical work of boundary marking between two nations.

Andrew Ellicott headed the joint U.S.–Spanish survey party that placed the stone on April 10, 1799, fixing the 31st parallel as the line between the Mississippi Territory and Spanish West Florida under the 1795 Pinckney Treaty, also called the Treaty of San Lorenzo. The stone was listed on the National Register of Historic Places on April 11, 1973.

The two inscriptions record the two sovereigns' claims: the north face reads "U.S. Lat. 31, 1799," the south face "Dominio De S.M. Carlos IV, Lat. 31, 1799" (Dominion of His Majesty King Charles IV). The stone thus carries both the American and the Spanish wording on a single block, a detail a visitor can read on the spot.

The stone stands in Ellicott Stone Historical Park, a 1917 park east of U.S. Route 43 about one mile south of Bucks. It sits near the west bank of the Mobile River, beside the James M. Barry Electric Generating Plant. The marker is a low block at ground level, so look down rather than up to read both faces.

The stone serves as the starting point for all federal land surveys in southern Alabama and Mississippi. From this single point, all survey lines and townships in the entire region were calculated.