The additional investment indicates that Japanese billionaire founder Masayoshi Son is committed to turning around the unprofitable U.S. wireless carrier that competes with AT&T, Verizon and T-Mobile US.
The Japanese Internet and telecom company purchased 45.8 million shares at an average price of $4.54 apiece in September, Bloomberg reported. In August, the carrier bought more than twice that many shares.
SoftBank is investing in Sprint, a telecom company that had booked losses in six of the past seven quarters. However, Sprint posted a third consecutive subscriber gain in its most recent quarter.
SoftBank, which is investing in the Indian e-commerce and green energy market in India, earlier said that it may make additional purchases of equity shares in Sprint while keeping its holdings to no more than 85 percent, which would make Sprint a target for delisting.
Recently Sprint said it will not participate in the upcoming 600 MHz low-band spectrum auction to be conducted by the FCC. At present, Sprint is investing significantly in its network modernization project to upgrade existing macro cell sites to support 800 MHz, 1900 MHz and 2.5 GHz for LTE in the U.S.