SoftBank and the Indian government are expected to make an announcement soon after final arrangements and made, the report said.
The company is expected to make the investment through a fund backed by Saudi Arabia's government, NHK said.
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.
And in April SoftBank teamed up with China's GCL System Integration Technology 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.