This video of Greg Olsen's 8-year-old son, TJ, telling his brother and sister that doctors found a heart donor for him will reduce you to tears in less than 15 seconds.

The ex-NFL star says the footage was taken exactly 1 week ago -- when the family was notified that a donor heart had become available for TJ.

"1 week ago, we were notified that we received an offer for a heart transplant," Olsen said ... "A special angel donor, somewhere, allowed Tj to share the news himself w/ Tate and Talbot."

FYI, TJ was born with a serious heart condition -- and underwent 3 open-heart surgeries before his 3rd birthday. Doctors recently advised the family his heart was in trouble and he needed a transplant, stat.

So, when Greg and his wife, Kara, got the news on Thursday, they let TJ inform his siblings -- and the reaction is pure love and joy.

"You know how I need a heart transplant?" TJ asks ... "and they've been trying to find one? Today, this morning -- they found one!"

The 3 kids hug and cheer and smile -- it's amazing. You can hear a voice off-camera remind the Olsens, "Just be gentle with him!"

TJ is over the moon as he tells his brother and sister the surgery will likely happen the next day (which it ultimately did).

And then, he drops even more good news -- "I got a covid test and it came back negative!"

Of course, TJ did undergo the procedure on Friday and doctors say it was a success. A few days later, TJ posted a video thanking everyone for their support.