SoftBank and Didi Chuxing will be leading Grab's $2.5 billion Series G funding round, the on-demand transportation and mobile payments company announced on Monday.
The lead investors will contribute up to $2 billion combined in what is one of the largest funding rounds in Asia, after Didi's $5.5 billion round and Ant Financial's $4.5 billion round.
Grab, Uber's biggest Southeast Asian rival, expects to close an additional $500 million from other investors.
The funding round comes less than a year since Grab confirmed raising $750 million in a Series F round.
The company claims a 95 percent market share in third-party taxi hailing and 71 percent market share in private vehicle hailing in Southeast Asia.
It operates in 65 cities across seven countries in the region and facilitates nearly 3 million rides a day, Grab claims.
Grab, which offers private car, motorbike, taxi, and carpooling services across seven countries, currently facilitates nearly 3 million rides daily.
Didi, which merged with Uber's China business in August 2016, first invested in Grab in 2015. The companies eventually formed a global anti-Uber alliance with their ride-sharing counterparts Lyft and Ola.
"[Didi and Grab] look forward to working together with communities and policymakers across Asia to fully embrace the extraordinary opportunities in the upcoming transportation revolution," Cheng Wei, founder and CEO of Didi, said in a statement.
Didi has also made separate investments in ride-sharing companies Ola and Lyft, and most recently led Brazil-based 99's $100 million round.
Didi had previously said it wants to create a "global mobility ecosystem".
SoftBank has also invested in Grab, leading its $250 million round back in 2014 as well as its $750 million round in September 2016.
The Japanese technology giant, which is also an investor in Ola and 99, is also expanding its footprint in the transportation industry, recently co-leading a $159 million round in technology company Nauto with Greylock Ventures.
Grab's valuation is expected to topple $6 billion following the closure of its current funding round.
The company said it will use the funds to support the growth of its mobile payments platform GrabPay. GrabPay Credits, a cashless top-up payments option, has grown 80 percent month-on-month since its launch in December 2016, the company claims.
In February, Grab announced its intention to invest $700 million in Indonesia over the next four years, supporting the Indonesian government's plan of becoming the region's largest digital economy by 2020.
As part of the investment, Grab said it would set up a research and development facility in the capital city of Jakarta focused on developing technology for the local market.