In a filing to the BSE, Firstsource said Rathen Investment sold 1,32,88,371 shares or 2.01 per cent stake in the BPO firm between November 27, 2013 and January 21, 2014.
Rathen earlier held 6.9 per cent share and following the closure of the deal, it will now own 4.87 per cent stake in Firstsource, it said.
Details about the buyer or the price at which the shares were purchased was not disclosed.
The deal could be worth about Rs 34.5 crore at yesterday's closing price of Rs 26 apiece on the BSE.
Firstsource had revenue of Rs 237.81 crore, while its profit stood at Rs 39.2 crore for the quarter ended September 30, 2013.
It is expected to announce its financial performance for the October-December 2013 quarter on January 31, 2014.