Didi Chuxing, which effectively ended Uber's ambitions in China, is one of the key backers.
Along with Japan's Softbank, Didi will invest up to $2bn. The other $500m will come from new and current investors.
Operating in seven countries, Grab is South East Asia's most popular ride-sharing firm.
Founded in Malaysia in 2012, it offers private car, motorbike, taxi, and carpooling services and holds 95% market share of "third-party taxi hailing" in the region, operating nearly 3 million daily rides.
Grab also runs a mobile payments business which customers can use to pay for rides and other services.
The company hopes the latest funding round - will allow it to expand both the transport and payments businesses.
Didi Chuxing dominates the Chinese ride-sharing market, and Softbank is one of Japan's largest technology companies run by entrepreneur Masayoshi Son.
"With their support, Grab will achieve an unassailable market lead in ridesharing, and build on this to make GrabPay the payment solution of choice for Southeast Asia," Grab chief executive officer Anthony Tan said.
The company has been steadily investing in the region to fight off competition from Uber and others. Earlier this year Grab bought Indonesian ecommerce business Kudo as part of a plan to invest $700m in the country, the firm's largest market.