SoftBank's April-June operating profit rose to 343.6 billion yen from 319.4 billion a year ago, helped by the boost from Sprint and growth in smartphone customers at its domestic telecoms business. That was better than the 320 billion yen average of three analysts' estimates surveyed by Thomson Reuters.
Sprint on Tuesday reported a smaller-than-expected quarterly loss and lower churn, or defections, a rare sign of improvement for a carrier that has struggled to compete against larger rivals AT&T and Verizon.
Son had made a rare appearance on Sprint's conference call to say SoftBank had no plans to sell its stake in the wireless carrier, a commitment he reiterated on Thursday.
"There was a time when I lost confidence in managing Sprint," he said. "Now I have no intention of selling."
Read MoreSoftBank: Why we changed investment focus
SoftBank has used cash from its telecoms business to fund investments in global mobile and Internet businesses, and the Sprint buy was aimed at growth outside Japan's sluggish economy.
Son said the latest buyback did not signal a change to its acquisitive strategy. SoftBank is also the largest investor in Chinese e-commerce giant Alibaba.
Jefferies estimates its 23 largest investments have delivered returns of 30 times with an average investment period of over 9 years. Excluding highly lucrative Alibaba, returns have been around 6 times.
SoftBank did not give a full-year outlook, but the market's average forecast prior to Thursday's announcement was for an annual operating profit of 1.04 trillion yen.
Based on such views, SoftBank shares have lately been trading below 8 times forward earnings estimates, lagging multiples of over 10 for Japanese telecoms rivals KDDI and NTT DoCoMo. Analysts have said the comparatively low valuations were due to concerns about Sprint.