4:14 PM PT -- NFL Commissioner Roger Goodellreleased a statement on Butkus' death.
"Dick Butkus was a fierce and passionate competitor who helped define the linebacker position as one of the NFL's all-time greats. Dick’s intuition, toughness and athleticism made him the model linebacker whose name will forever be linked to the position and the Chicago Bears.
"We also remember Dick as a long-time advocate for former players, and players at all levels of the game. The Dick Butkus Award and his foundation honored achievement on the field and service to the community among high school, college and NFL linebackers. Dick was a champion of clean sports as his “I Play Clean” campaign helped raise awareness about the dangers of steroid use among high school athletes.
"We send our deepest condolences to the Butkus family, the Bears organization and the many fans and people he impacted throughout his life."
3:46 PM PT -- The Butkus family just released a statement ... saying Dick died in his sleep overnight.
"The Butkus family is gathering with Dick's wife Helen. They appreciate your prayers and support."
The Bears added ... "Dick was the ultimate Bear, and one of the greatest players in NFL history. He was Chicago's son. He exuded what our great city is about and, not coincidently, what George Halas looked for in a player: toughness, smarts, instincts, passion and leadership."
The statement continued ... "He refused to accept anything less than the best from himself, or from his teammates.
"Dick had a gruff manner, and maybe that kept some people from approaching him, but he actually had a soft touch. His legacy of philanthropy included a mission of ridding performance enhancing drugs from sports and promoting heart health.
"His contributions to the game he loved will live forever and we are grateful he was able to be at our home opener this year to be celebrated one last time by his many fans."
NFL legend Dick Butkus -- one of the greatest linebackers of all time -- has died, TMZ Sports has learned. He was 80 years old.
We're told someone went to check on Butkus at his home on Thursday ... and he was found unresponsive.
Our sources tell us ... paramedics responded to a call for a patient in cardiac arrest at Butkus' residence in Malibu at 12:51 PM. He was pronounced dead at the scene.
Butkus is among the most intimidating players to ever step onto a football field -- coming in at 6'3", 245 pounds, he was a defensive standout at the University of Illinois before joining his hometown Chicago Bears in 1965 ... going No. 3 overall in the NFL Draft.
He played his entire professional career with the Bears from 1965 to 1973 ... racking up every individual award imaginable -- six All-NFL selections, two Defensive Player of the Year honors and eight Pro Bowls.
Butkus earned many nicknames due to his style of play .. including The Maestro of Mayhem, The Enforcer, The Animal and The Robot of Destruction.
Butkus was named to the league's All-Decade teams in the '60s and '70s ... as well as the 75th and 100th Anniversary teams.
Butkus -- whose career was shortened due to injuries -- had 1,020 tackles, 22 interceptions and 27 fumble recoveries over the span of nine seasons. He was enshrined into the Pro Football Hall of Fame in 1979, his first year of eligibility.
In 1985, the Butkus Award was established to honor the top linebackers at the high school, college and professional levels.
The Bears retired his No. 51 jersey in 1994.
Off the field, Butkus was known for starring in commercials for Miller Lite, Prestone, Schick and Visa ... as well as dozens of films and TV shows, including "Any Given Sunday," "Hang Time," and "The Last Boy Scout." He was also an analyst for CBS.
Butkus continued to follow the NFL in retirement -- in fact, he posted his weekly picks on his Instagram just hours ago.
Butkus is survived by his wife, Helen, and three children.