New Yorkers pretty soon could be whipping out smartphones instead of cash or credit cards to pay their yellow-cab fares.

The city is seeking a software developer to design an app that lets passengers pay with their cellphones — a method that could pave the way for frequent-rider discounts, officials said.

The Taxi & Limousine Commission yesterday issued a request for proposal — which city agencies release when seeking private partners — calling on techies to submit their best ideas for an easy-to-use taxi app.

The request also encourages developers to come up with other smartphone proposals that could be implemented affordably — like alerts for available cabs and an app to encourage ride sharing.

Even though most New Yorkers are just getting used to paying by credit card — which came about system-wide only four years ago — the city is convinced smartphone payments will be a boon to riders.

“Paying via smartphone could enable passengers to receive receipts by e-mail . . . and potentially allow them to accumulate ‘points’ or ‘rewards’ through a frequent-riders program,” according to the request posted on the TLC’s Web site.

Letting riders pay by phone will help keep the city competitive, said TLC Commissioner David Yassky.

“Smartphones are poised to become a payment method of choice for many people over the next few years,” he said, adding, “We want to keep NYC taxis in the forefront of consumer-friendly technology.”

No other city in the nation has its entire taxi fleet wired for phone payments.

In order to be approved for use, the app must be free of charge to both customers and the city, according to the request.

The developer could profit from advertising or credit-card fees.

Fare-ly easy

* A potential new city app would let riders use smartphones to pay their fare

* It could also include a rewards program — similar to a frequent-flier program

* City also wants apps to help riders find cabs and easily share trips