SoftBank and the Indian government are expected to reach a formal agreement soon after final arrangements are made, the report said without naming its sources.
The company is expected to make the investment through a fund backed by Saudi Arabia's government, NHK said. Saudi Arabia is the largest investor in SoftBank's Vision Fund, which raised over $93 billion last year.
A SoftBank spokesman declined to comment.
In 2015 SoftBank pledged to invest $20 billion in Indian solar projects with a goal of generating 20 gigawatts (GW) of energy as the majority partner in a joint venture with India's Bharti Enterprises and Taiwan's Foxconn.
Last month SoftBank secured 200 megawatts of solar capacity at an auction in the southwestern state of Karnataka.
In April it teamed up with China's GCL System Integration Technology Co Ltd on a $930 million Indian solar energy venture.
India has set a target to achieve an operational solar power capacity of 100 GW by 2022, five times current levels, under Prime Minister Narendra Modi's renewable energy strategy.
SoftBank's Vision Fund has exposure to solar energy through its investment in the world's largest such project in Saudi Arabia announced in March.
(Reporting by Sam Nussey and Kaori Kaneko, Editing by Stephen Coates)