Lesnar was looking to get back into a title fight tonight, but the knees of Alistair Overeem proved to be too much for the former UFC champ.
Lesnar's announcement may have been shocking to most people.
But unlike many MMA stars, Lesnar picked the right time to retire.
Continue reading to find out why this was Lesnar's time to go.
If there's one thing Brock Lesnar wasn't fighting for, it's money.
Lesnar was the UFC's biggest draw and earned well over a million dollars per fight.
Even before Lesnar fought in the UFC, he earned a lot of money from his days in the WWE.
A lot of fighters' main motivation is earning a paycheck to support their families.
Lesnar, on the other hand, is very stable financially without fighting.
Although Lesnar can paint himself as the bad guy at times, he is a family man at heart.
Lesnar's family almost lost him twice due to diverticulitis.
That, along with fighting in one of the most dangerous sports in the world, had to put a lot of stress on them.
Lesnar is a very private person and can now focus all of his time on the people who love him the most.
Brock Lesnar accomplished a lot more in eight fights than most fighters do in their entire careers.
Lesnar won the UFC heavyweight championship, became the UFC's biggest draw and came back twice from a life-threatening illness.
Whether you love or hate Lesnar, you have to respect him for doing more than many believed he could do in the sport.
Brock Lesnar's body has been through a lot the past two years.
Lesnar has had two bouts with a life-threatening illness, had major surgeries to correct the issue, had to change his diet and had to get back in shape to come back to the UFC.
MMA is one of the most demanding sports in the world.
The training alone takes a lot out of the fighters and often leaves them less than 100 percent on fight night.
This is a chance for Lesnar to give his body some much-needed rest.