Sprint needs that investment to help it pay for a network upgrade and step up competition with No. 1 and No. 2 rival providers AT&T and Verizon. The money would also help pay for the Clearwire buyout.
Clearwire, in which Sprint already owns a majority stake, is important to the merger by SoftBank and Sprint because it holds a large amount of wireless airwaves, or spectrum, which will help Sprint compete against its bigger rivals.
"We are one step closer to a stronger Sprint, which will better serve consumers, challenge the market-share leaders and drive innovation in the American economy," Sprint CEO Dan Hesse said in a statement.
Clearwire's minority shareholders will vote on Sprint's buyout plan on July 8. Several minority shareholders had launched a boisterous campaign seeking a higher bid from Sprint and siding with rival bidder Dish Network but large player Crest Financial dropped its opposition on Wednesday after Sprint sweetened its offer.
SoftBank also had to up its offer for Sprint to shake off a rival one from Dish's billionaire founder Charlie Ergen.
(Read More: Sprint Raises Clearwire Bid, Wins Key Investor Support)
"SoftBank's investment in Sprint will bring innovation and increased customer focus, which will enable us to begin creating a true competitor in a market dominated by two companies," SoftBank's CEO Son said in a statement.
The FCC's review focused on whether the two related deals were in the U.S. public interest and found them likely to result in improved mobile broadband access, greater innovation and potentially lower prices for broadband users, "including rural, low-income, and minority consumers."
"The proposed transactions are unlikely to result in competitive or public interest harms," the FCC's order said. "We are not persuaded by requests that the commission impose buildout requirements, require divestitures of spectrum, or request investment commitments by the applicants."
AT&T on Wednesday, when sources said the FCC had the votes to approve the deals, decried the agency's refusal to satisfy Verizon's and others' request to use this review to also address how it views and calculates spectrum ownership.
The FCC's order said the transaction was not an appropriate place for the review of the so-called spectrum screen, because from FCC's perspective no airwaves were changing hands. SoftBank owns no U.S. spectrum and the FCC has long attributed Clearwire's spectrum to Sprint for screen purposes.