BART Watch is free to download from the agency's website www.bart.gov, as well as the Apple App Store and Google Play.
"This new technology will allow us to partner with our riders so they can 'see something, say something' and help us keep the system safer and more secure," BART Police Chief Kenton Rainey said in a statement.
The app will also be offered in Spanish and Chinese.
Riders will be able to use BART Watch to send a text message of what they are seeing. There is an option to attach a picture to the text message.
The app also allows riders to call BART police directly or send text reports anonymously.
In areas with no cellular service, a text message sent will be delivered once the rider goes to an area with service.
The app has pull-down menus that lets riders choose which report they want to send, including a crime in progress, an illegally parked vehicle, disruptive behavior, a robbery or theft, an unattended bag or package and vandalism.
The reports will go directly to BART police dispatch to be prioritized for response.
BART police will also use the app to send alerts out to riders.
BART officials were planning a news conference at 11 a.m. today at the Powell Street station in San Francisco to discuss the launch of the app.