It is the first mass transport system in the US to adopt the technology.
Portable scanners will be used to screen passengers as they enter stations, without them having to pass through a security checkpoint.
Authorities said the screening would be "voluntary", but those refusing a scan will not be allowed to travel.
The Los Angeles County Metropolitan Transportation Authority (LACMTA) said it had ordered equipment from UK manufacturer Thruvision.
The company's equipment is not currently used on UK public transport, but it has been trialled at the Farnborough Airshow.
The scanners have a wide field of view and can screen passengers as they ride an escalator or enter through ticket barriers.