By P.R. Venkat

Abu Dhabi-based sovereign investor Mubadala Investment Co. has agreed to invest 90.90 billion rupees (US$1.21 billion) in Jio Platforms Ltd., joining the list that includes Facebook Inc. and other foreign firms eager to buy a stake in the Indian telecommunications and technology giant.

Mubadala's Investment will give it a 1.85% stake in Jio Platforms, which has an equity value of INR4.91 trillion, Reliance Industries said Friday.

With this investment, Jio Platforms has raised INR876.66 billion in less than six week from various big multinational companies.

These investments include that by Facebook Inc., Silver Lake, Vista Equity Partners, KKR and General Atlantic L.L.C.

In April, Facebook agreed to invest $5.7 billion for just under a 10% stake in the Mumbai-based telecom firm.

Reliance in recent years spent more than $30 billion to build from scratch its upstart mobile operator, Reliance Jio Infocomm Ltd., which is part of Jio Platforms. Reliance has said that it is aiming for it to be net debt-free by next year.

Jio has attracted more than 388 million subscribers with inexpensive mobile-data plans and is now moving into e-commerce, as legions of people in the country of 1.3 billion begin transacting online for the first time.

Reliance Industries has investments spanning media, retail and natural resources.

Morgan Stanley advised Reliance Industries on the deal.

Write to P.R. Venkat at venkat.pr@wsj.com