Sprint's shares were up 8.2 percent at $4.20 in extended trading on Wednesday.
SoftBank, which owns about 80 percent of Sprint, said it purchased additional shares of Sprint through its wholly-owned subsidiary Galaxy Investment Holdings at a cost of about $87 million.
SoftBank said, as a result of the purchase, it does not contemplate that its ownership of Sprint's outstanding common stock will increase to 85 percent or more.
SoftBank had lost confidence in Sprint and was considering selling it as Sprint had been burning through cash because of monthly leasing plans that require U.S. wireless carriers to pay vendors for devices up front.
However, after Sprint last week reported a smaller-than-expected quarterly loss, as it added a net 675,000 customers helped by promotions and offers such as doubling data capacity, SoftBank assuaged investors' concerns by saying it had no plans to sell its stake in the wireless carrier.
SoftBank's Chief Executive Officer Masayoshi Son also made a rare appearance on Sprint's earnings call and said that SoftBank would set up a leasing company with other partners to finance payments of devices leased by Sprint customers.
SoftBank said on Wednesday it was "enthusiastic" about Sprint's prospects and working closely with Sprint on its "network strategy to enhance Sprint's competitiveness and reduce its capital expenditures and operating costs."
At Wednesday's close of $3.88 on the New York Stock Exchange, Sprint's shares had fallen about 7 percent this year.