The baseball world is rallying behind Chicago Cubs great Ryne Sandberg ... who just revealed Monday he is receiving treatment after being diagnosed with cancer.

The 64-year-old Hall of Famer shared the health update on his social media just minutes ago ... saying, "Last week, I learned that I have been diagnosed with metastatic prostate cancer. I have begun treatment, and I am surrounded by my loving wife Margaret, our incredibly supportive family, the best medical care team, and our dear friends."

Despite the battle ahead, Ryno said he will remain in good spirits and "fight to beat this."

"Please keep us in your thoughts and prayers during this difficult time for me and my family," he added.

The comments section under Sandberg's post has been flooded with words of encouragement from fellow Cubs like Dexter Fowler and David Ross ... as well as countless others.

Sandberg played the majority of his career with the Cubbies from 1982-1997 ... earning 10 All-Star appearances, nine Gold Gloves, seven Silver Sluggers and National League MVP honors in 1984.

His No. 23 jersey was retired in 2005 ... the same year he was enshrined in Cooperstown.

Following his playing career, Ryno managed the Philadelphia Phillies from 2013-15.

The Cubs addressed Sandberg's diagnosis in a statement ... saying, "We cannot imagine how incredibly tough it is right now for Ryne and his family, but we do know Ryne is one tough competitor and a winner."

"We are rallying around his family with locked arms as they begin their journey to conquer this battle with cancer."