The 65-year-old Allen shared the news Monday, writing that his team of doctors has started treatment and that he plans on fighting the disease aggressively.
"A lot has happened in medicine since I overcame this disease in 2009," Allen wrote. "My doctors are optimistic that I will see good results from the latest therapies, as am I. I will continue to stay involved with Vulcan, the Allen Institutes, the Seahawks and Trail Blazers, as I have in the past. I have confidence in the leadership teams to manage their ongoing operations during my treatment. I am very grateful for the support I've received from my family and friends. And I've appreciated the support of everyone on the teams and in the broader community in the past, and count on that support now as I fight this challenge. Go Seahawks! Go Blazers!"
Allen, who is a co-founder of Microsoft, has owned the Seahawks since 1997 and the Blazers since 1988. He's also the co-owner of the MLS' Seattle Sounders FC.
The Seahawks last week announced a change in their organizational structure, with Chuck Arnold replacing Peter McLoughlin as team president. Arnold will represent Allen at league functions, as McLoughlin did.
"We know that Paul's been through this before and he battled like a real warrior last time and made it through it," Seahawks coach Pete Carroll said Monday. "We're all pulling for him, too, and we're grateful that all he stands for in this community and for all of us. So we're going to join together and help him battle along with this process that he's going through, that he knows very well. So we're thinking only good thoughts."