The move is part of the US city's attempts to adapt to the hygiene challenges of the Covid-19 pandemic - reducing the need for staff to handle cash and reassuring travellers that they are lessening the risk of infection.
Fares for St Louis Metro Transit's MetroBus and MetroLink light rail services, which serve parts of eastern Missouri and south-western Illinois, can be bought in this way.
Adding mobile ticketing to the app - which already allows users to plan and track their trip - is via payment provider Masabi’s Justride software development kit.
“Mobile ticketing not only supports the health and safety of our riders and operators during this pandemic, but also provides all of our customers with a better transit experience," insists Jessica Mefford-Miller, executive director of Metro Transit.
Tickets can be "visually validated by operators and fare inspectors from a safe distance".
App users can buy full-fare and reduced-fare passes, and keep them in a Transit wallet until they are ready to travel.
“We want to make it as easy as possible for people to ride public transit," said David Block-Schachter, chief business officer at Transit.
"During the coronavirus response, transit agencies are working to keep riders and employees safe by reducing close physical contact, and this solution helps them do that."
Transit says that more than a dozen US transportation agencies are now offering mobile ticketing with the app, including Cincinnati, Denver and Las Vegas.