The Tokyo-based mobile giant said the project aimed to generate 20GW (gigawatts) of energy through solar and wind power plants.
India relies on highly pollutant coal for 60% of its energy needs.
But prime minister Narendra Modi wants clean energy to help bring electricity to 300 million people without power.
SoftBank's founder, Masayoshi Son, made the latest announcement in Delhi, flanked by Sunil Bharti Mittal, chairman of Bharti Enterprises, India's biggest wireless provider.
"With this partnership, our goal is to create a market-leading clean energy company, to fuel India's growth with clean and renewable sources of energy," Mr Son said in a press release.
Mr Son said his company would help manufacture solar equipment in India, adding that SoftBank has invested $1bn in the country in the last nine months.
"The sunshine in India is twice that of Japan. Cost of land and labour and maintenance is also half that of Japan. So then, the efficiency of solar power generation is likely to be four times compared to that in Japan," he told the CNBC TV18 channel.
Mr Modi has insisted that his commitment to increase India's renewable energy supplies is not aimed at "impressing the world" following international pressure to cut greenhouse gas emissions.
Japanese firm SoftBank will team up with Taiwan's Foxconn and India's Bharti Enterprises to invest $20bn (Â£12bn) in solar projects in India.