The JV deal caps off a 12-month search by Robust for an Indonesian partner to satisfy the future statutory requirement for local participation.
After Robust took full ownership of the project in April last year, the board decided to seek a local partner with in-country experience, government relationships and a strong track record.
PT KSP is part of Indonesia's Salim Group, which has diversified business interests across the country.
As part of the deal, a representative of Salim will join the Robust board.
Robust managing director Gary Lewis described the new partnership as transformational.
"We have been working for 12 months to identify and secure a strong Indonesian partner that has the experience, an understanding of domestic politics and bureaucracy, and the skills to help us take the Romang Island project through to successful development," he said.
The cash injection will give Robust's Indonesian subsidiary, PT Gemala Borneo Utama, a cash balance of more than $60 million, which will fund the feasibility and development stages of the project.
Robust already has six owner-operated diamond drill rigs on Romang Island, with the aim of calculating a maiden mineral resource estimate.