The route is signed east–west, but forms a half-loop, with most of its western segment running strongly southeast slightly inland of the Piscataqua River and the state border with New Hampshire, its central skirting the Piscataqua generally easterly, and its eastern strongly northeast slightly inland from the Atlantic Ocean.

For its first mile SR 103 heads south-southwest, until crosses Sturgeon Creek, a tidal inlet of the Piscataqua River, the state border with New Hampshire.

SR 103's first intersection is two-thirds of a mile later at Dennett Road, where it T's right and passes underneath U.S. Route 1 Bypass.

SR 103 continues southeast, intersecting US 1 just north of the Memorial Bridge connecting Kittery with Portsmouth, New Hampshire.

There it wends eastward along the outer harbor of the Piscataqua through historic Kittery Point before veering northeast at Gerrish Island toward York.