The Westside Purple Line Extension (formerly known as the Westside Subway Extension) is the extension of the Purple Line from its current terminus at Wilshire/Western, nine miles to Los Angeles' "second downtown" through the neighborhoods of Beverly Hills, Century City, and Westwood. The new line will ultimately add seven stations upon the completion of all three phases of the project. Section I of the project also includes the procurement of 34 new heavy rail vehicles compatible with the existing system and improvements to existing Division 20 Rail Maintenance and Storage Yard to accommodate additional vehicles. The extension is currently slated to be built in three sections by the design-build delivery method: Section 1 is a 3.9-mile long segment along the Miracle Mile to Wilshire and La Cienega Boulevards (including three stations), Section 2 is 2.6 miles from La Cienega to Century City, and Section 3 is a 2.9-mile section through Westwood to the Department of Veterans Affairs (VA) Hospital. The extension will consist of twin tunnels approximately 20 feet wide, located 50-70 feet underground.

LACMTA commissioned an economic impact study from the Los Angeles Economic Development Corporation (LAEDC) for the 12 projects connected to the Measure R local retail sales tax.  Per LAEDC estimates, the projects will produce an economic impact of approximately $3.5 billion in direct, indirect, and induced business revenues.  The Westside Purple Line Extension, Section 1 is expected to create 22,500 jobs in the construction phase alone. Once completed, the project is expected to pull over 30,000 drivers off of the road, saving approximately 65 million vehicle miles travelled per year. TIFIA loans provide significant benefits to LACMTA.  TIFIA interest rates are lower, which will result in financing cost savings of approximately $100 million, and TIFIA-secured loans have allowed LACMTA to maximize debt capacity.  The $856 million TIFIA loan for Westside Subway and other LACMTA projects have helped accelerate infrastructure investment in the region and deliver significant economic advantages.