The 34-year-old golfer made the heartbreaking announcement on his Instagram page Thursday ... saying his mom, Adenil "Dening" Day, died on Wednesday night five years after she was diagnosed with lung cancer.

"We are heart broken but incredibly grateful for the gift we had in her living with us for the last almost two years full time," Day said. "She fought so hard until the very last breath."

According to the PGA Tour, Dening passed away peacefully with Jason and other family members by her side.

"I am forever indebted to her for the sacrifices she made for me to be successful, and for the person she helped me to become," said Day, who pulled out of the Arnold Palmer Invitational earlier this week to be with his mother in her final days.

"We will miss her so much."

Tragically, Dening is not the first parent Jason has lost to cancer ... his father passed away from stomach cancer when Jason was just 12 years old.