Paul Allen, the billionaire co-founder of Microsoft Corp. who went on to invest his fortune in ventures from the Seattle Seahawks to brain research to the arts, died Monday, two weeks after revealing the return of a cancer he had received treatment for years ago. He was 65 years old.

Mr. Allen’s holding company, Vulcan Inc., announced his death.

Mr. Allen rose to fame as a pioneer in personal computing. In 1975, he joined with his high-school friend Bill Gates to found Microsoft, a software company that in the ensuing decades went on to dominate the PC industry.

“I am heartbroken by the passing of one of my oldest and dearest friends,” Mr. Gates said in a statement. “Paul was a true partner and dear friend.”

In a story that has become part of Silicon Valley lore, Mr. Allen raced to share with Mr. Gates an article about a computer called the MITS Altair 8800 that appeared in the January 1975 issue of Popular Electronics magazine. Mr. Gates, a student at Harvard University at the time, dropped out to start the new software company with his friend.