Paris Métro Line 18 is one of four new lines of Grand Paris Express, a major expansion project of the Paris Métro. Currently under construction, it will link Orly Airport to Versailles via Massy-Palaiseau, the Saclay Plateau, and Saint-Quentin-en-Yvelines. The line will be 35 kilometers (22 mi) long and will be fully automated (along with all Grand Paris Express lines). Subsequently, it is planned to be extended by about 15 kilometers (9.3 mi) from Versailles to Nanterre via Rueil-Malmaison.
It is expected to be completed after 2030. Its first section, from Orly Airport to the Saclay plateau (Christ de Saclay), is scheduled to open in 2026 between the stations of Massy-Palaiseau and Christ de Saclay, in 2027 between Massy-Palaiseau and Orly Airport, and in 2030 between Christ de Saclay and Versailles Chantiers. One of the line's aims is to serve the technological and scientific development cluster of Paris-Saclay and the campus of Paris-Saclay University.