The Red Sox made the announcement on behalf of Lester's family shortly after 6 p.m. ET on Friday.
The specific condition is known as anaplastic large cell lymphoma, and Lester is expected to begin treatment within the coming week.
"Jon and his family wish to thank all those involved in his care at Massachusetts General Hospital," the family said through a statement. "Our gratitude also extends to the Red Sox organization, which has provided Jon and his family much needed support during this ordeal.
"We ask that you respect our need for privacy during this difficult time."
Lester, in his first Major League season, is 7-2 with a 4.76 ERA in 15 starts for the Red Sox.
Red Sox players, coaches and uniformed personnel were informed of Lester's condition during a meeting, called by manager Terry Francona, about one hour before Friday's game.
"Before the game, we met to make sure everyone understood what's going on," Francona said. "Other than that, it's such a private matter, and the Lesters have asked us to try to keep it that way. Our prayers are with him and his family. I understand that there's so much interest in everything that happens with the Red Sox. I think everybody can understand the need to keep some privacy and some respect."
Jonathan Papelbon, who sustained a shoulder injury and came out during the ninth inning Friday night, offered perspective on his condition compared to the battle ahead of Lester.
"It's tough and with my situation, it makes it seem like no big deal, because you've got so much bigger things in life going on," Papelbon said. "Just to get a small injury like I got right now makes it miniscule compared to other things that Jon's got going on with him right now."
Papelbon said the team went as far as to look ahead to a possible return for Lester in 2007.
"Good thing with him, they say it's curable and treatable and they're expecting him to be at Spring Training with us next year," Papelbon said. "Obviously, we all send our support and prayers out to him, and hopefully he'll be there."
Mike Lowell, diagnosed with testicular cancer in 1999, felt for his teammate but also sounded an encouraging tone.
"I have an idea, but you know it seems like two different scenarios," Lowell said. "I don't know what's in store for him. The feelings I can understand, because it's a time where he's going to want to be with his family and not really have too many outside distractions."
Lowell said Lester would likely go through an emotional time over the next couple of days, sorting out emotions.
"Everything for me happened so fast," Lowell said. "I didn't really have time to soak it in. It was a matter of two days. But you're scared. That's a normal feeling, but the fact that it's something he can recover from is very positive.
"I think the best thing is that if it's something where the doctor's diagnosis is it's treatable and curable, take it for what it's worth. Get well and then worry about baseball later."
Mike Petraglia is a contributor to MLB.com This story was not subject to the approval of Major League Baseball or its clubs.