The report, citing an unnamed person with “knowledge of the negotiations” said that the deal being negotiated is worth more than $12 billion.
Reuters and Japanese broadcaster NHK also reported news of a possible acquisition deal.
Sprint spokesman John Taylor declined to comment on the matter, saying “we do not comment on rumor or speculation.”
The third-largest carrier in the U.S., Sprint has been unable to catch AT&T and Verizon in smartphone and tablet sales as it builds out its own high-speed 4G LTE network.
Predictions that Sprint was looking for a merger have been on the rise since fourth-place T-Mobile announced it was making a bid for MetroPCS last week. But speculation had largely centered on Sprint making its own acquisition bid for T-Mobile, MetroPCS or another, smaller carrier.
Softbank, the Japanese firm, is in a similar position as its country’s third-largest firm. It was the first in Japan to carry Apple’s iPhone.
The carrier has a history of high-profile acquisitions. Earlier this month, the company announced it would buy the Japanese firm eAccess in order to expand its own LTE network. In 2006, it acquired the Japanese arm of the Vodafone Group.