They will be able to tap their contactless card or device (smartphone or watch) on the ticket machine when boarding the bus and tap off on the tap off reader as they leave to pay for their travel.
Fares purchased using the pay as you go system will be capped at daily and weekly levels varying by area to offer the best value to customers.
Pay as you go users will be able to track their journeys and spending by signing up for a Stagecoach account.
Those who use the contactless pay as you go system will need to make sure they tap off at the end of their journey. If they forget, they could be charged the cost of a single fare to the end of the bus route.
The payment system is for adult fares only.
Martin Gibbon, managing director of Stagecoach South Wales, said: “Our pay as you go system makes jumping on a Stagecoach bus even easier, with more flexibility. Offering great value with capped daily and weekly tickets.
“We’ve listened to our customers, previously if they forgot to tap off, they’d incur a flat £10 charge. We’ve now improved the system to change this.”
Cabinet Secretary for Transport and North Wales, Ken Skates said: ““This is great news for Stagecoach passengers. Pay as you Go provides a simpler and fairer way to travel and in turn will encourage more people to travel by bus.”
Capped fares vary on different routes.