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 'ville nouvelle' of Cergy, and another in the East, to the ville nouvelle of Marne-la-Vallée. The two latest extensions of the line were to Cergy-Le Haut and Disneyland Paris.