Allen, 65, said he has begun treatment and that he and his doctors are optimistic he will once again beat the disease.
"Paul's been through this before ... he had a positive outcome," Seahawks coach Pete Carroll told reporters Monday. "We're all pulling for him, too. We're all grateful for all that he stands for in this community and for all of us. So we're going to help him battle along with this process that he's going through that he knows very well."
Here is the complete statement Allen posted on his personal website:
I learned recently that the non-Hodgkin's lymphoma that I was treated for in 2009 has returned. My team of doctors has begun treatment of the disease and I plan on fighting this aggressively.
A lot has happened in medicine since I overcame this disease in 2009. 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.