The RER A is one of the five lines in the RER rapid transit system serving Paris, France.

The line runs from the western terminuses Saint-Germain-en-Laye (A1), Cergy Le Haut (A3), and Poissy (A5) to the eastern terminuses Boissy-Saint-Léger (A2) and Marne-la-Vallée – Chessy (A4).

Line A is one of the world's busiest lines with over 1,200,000 passenger/day. Line A is formed from the connection across Paris of the Saint-Germain-en-Laye-Nanterre line in the west to the Vincennes – Boissy-St-Léger line in the east. Two branches were added in the West, to Poissy and the new town of Cergy-Pontoise, and another in the East, to the new town of Marne-la-Vallée. The two latest extensions of the line were to Cergy-Le Haut and Disneyland Paris.