NEW DELHI—Paytm, India’s biggest mobile-payments firm, now wants to dethrone Facebook Inc.’s WhatsApp as the most popular messaging service in the South Asian country.

The Noida, India-based company is preparing to introduce messaging functionality into its app which has more than 225 million users, according to a person familiar with the matter.

That could allow Paytm, which in May raised $1.4 billion from Japan’s SoftBank Group Corp., to woo some users from WhatsApp, driving usage on in its fast-growing platform.

Chat functionality could also provide Indian businesses with an official channel through which to interact with customers, a feature WhatsApp founders said last year they were working on.

A spokeswoman for WhatsApp, which says 200 million of its 1.2 billion users are in India, declined to comment. The company is exploring adding a payment option for India and has advertised for a digital transactions lead in the country.