The deal, which is expected to close in the second quarter of 2016, will lead to a creation of an operator with 118 million subscribers.
Sistema Shyam Teleservices Ltd (SSTL) will hold about 10 per cent stake in RCom and pay off its existing debt before closing the deal.
While the two companies did not disclose financial details of the deal, industry sources said SSTL's equity stake has been valued at USD 290-300 million at current prices.
This is the first consolidation in a highly competitive telecom sector. While RCom is the country's fourth largest operator with 110 million subscribers, SSTL is positioned at number nine with close to nine million users.
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SSTL offers mobile telephony services under MTS brand across nine telecom circles in the country.
"RCom will acquire approximately 9 million customers and approximately Rs 1,500 crore of annual revenues by virtue of the transaction," the company said in a statement.
The deal will give RCom access to spectrum or airwaves in the 850 Mhz band, which can be used for 4G services that it plans to start by the year-end.
While SSTL will pay off its existing USD 500 million debt, RCom will assume the liability to pay the government installments for SSTL's spectrum, amounting to Rs 392 crore per annum for the next 10 years.
Sistema may get up to USD 300 million in cash from RCom if the government were not to agree on the fee payment.
Russian tycoon Vladimir Evtushenkov-controlled AFK Sistema currently holds 56.68 per cent stake in SSTL while Russian government owns 17.14 per cent interest. Shyam Group has 23.98 per cent stake and the rest is owned by small investors.
RCom had last month announced partnership with Reliance Industries' telecoms unit, Jio, to trade and share spectrum.
SSTIL had scaled back operations in 2013 when it had to buy new spectrum after its earlier permits were among those cancelled by the Supreme Court.