Sprint Nextel Corp. has struggled for half a decade with deep losses and a huge debt load. Now, help may be on the way from Japan.

Japanese phone and Internet company Softbank Corp. is in talks to buy a significant stake in the ailing U.S. wireless carrier, Sprint said. A deal would offer a multibillion-dollar lifeline that could help Sprint finance future mergers of its own and better compete with its much bigger and richer rivals.

The...