Allen co-founded Microsoft alongside Bill Gates in 1975.
Microsoft co-founder and Seahawks, Trail Blazers and Seattle Sounders owner Paul Allen died on Monday at the age of 65 from "complications of non-Hodgkin's lymphoma."
"Paul's family and friends were blessed to experience his wit, warmth, his generosity and deep concern," Allen's sister Jody said in a statement. "At this time of loss and grief for us—and so many others—we are profoundly grateful for the care and concern he demonstrated every day."
Allen announced he had non-Hodgkin's lymphoma on Oct. 1.
The Trail Blazers were Allen's first professional sports franchise that he owned, purchasing them in 1988. Allen bought the Seahawks in 1996. Seattle has reached the Super Bowl three times since then, and the team won Super Bowl XLVIII.
NFL commissioner Roger Goodell released a statement on Allen's death.
"Paul Allen was the driving force behind keeping the NFL in the Pacific Northwest. His vision led to the construction of CenturyLink Field and the building of a team that played in three Super Bowls, winning the championship in Super Bowl XLVIII," Goodell wrote. "Our league is better for Paul Allen having been a part of it and the entire NFL sends its deepest condolences to Paul's family and to the Seahawks organization."
NBA commissioner Adam Silver also provided a statement on Monday evening.
"Paul Allen was the ultimate trail blazer – in business, philanthropy and in sports," Silver wrote. "As one of the longest-tenured owners in the NBA, Paul brought a sense of discovery and vision to every league matter marge and small... He was a valued voice who challanged assumptions and conventional wisdom and one we will deeply miss as we start a new season without him."
Allen purchased the MLS's Seattle Sounders in 2009. They currently share CenturyLink Field with the Seahawks.
Allen co-founded Microsoft with Bill Gates in 1975. He also developed a string of other businesses and philanthropies, including the Allen Institute for Brain Science.