Greg Olsen is mourning a heartbreaking loss … the former NFL star revealed his older brother, Chris, has died after a battle with brain cancer.

The longtime Panthers tight end and current FOX broadcaster shared the devastating news in an emotional Instagram tribute … explaining Chris lost his fight with glioblastoma — one of the most aggressive forms of cancer — after putting up a relentless fight.

“Love you brother,” Olsen wrote with a heart emoji.

Olsen made it clear his brother never stopped battling, praising Chris’ toughness and spirit throughout the illness … saying the disease may have taken a physical toll, but it never broke who he was as a person.

The 40-year-old former Pro Bowler also thanked friends, fans and supporters who stood by his family during Chris’ health battle … showing appreciation for the prayers and messages they’ve received during the incredibly difficult time.

Chris, who was just 42 years old, was known for his larger-than-life personality that made everyone feel his love. As his condition worsened in recent weeks, family and friends traveled from across the country to visit him in Little Rock, where he was receiving care.

Greg has previously spoken about their tight bond … with the brothers leaning on each other constantly as Chris fought the disease.

The loss is another emotional blow for the Olsen family, which has been open about serious medical challenges over the years … including Greg’s advocacy work surrounding health issues affecting loved ones.

Chris’ death leaves family, friends and supporters remembering him as a fighter who refused to let cancer define him.