Le Pecq's territory is astride the two banks of the Seine and includes a small island, Corbière.
It borders the communes of Saint-Germain-en-Laye and Mareil-Marly in the west, Montesson and Le Mesnil-le-Roi (Carrières-sous-Bois district) to the north, Croissy-sur-Seine and Le Vésinet in the east and Marly-le-Roi and Le Port-Marly to the south.