Pickens tweeted "Big news today" in announcing his Mesa Vista Ranch, covering more than 100 square miles (259 sq. kilometers), was for sale. The spread is about 90 miles (144 kilometers) northeast of Amarillo.
"I'm proud of our conservation efforts that have taken place over decades and hope the next owner has the same passion," Pickens tweeted Wednesday. He did not immediately identify any potential buyers.
Pickens, 89, has had health concerns that include a series of strokes.
Pickens on Valentine's Day, 2014, married his fifth wife, Toni Brinker, in a ceremony at the ranch. The couple recently divorced.
Pickens this fall put his Dallas mansion on the market for $5.9 million.
"My Alva Court property in Dallas is a wonderful home with some incredible neighbors," Pickens told The Dallas Morning News. "But as an 89-year-old single man, I've just got too much house on my hands."
Pickens built one of the country's biggest independent oil companies, Mesa Petroleum, before moving on to BP Capital and founding clean transportation fuels company Clean Energy. He's traveled the country championing wind power and other alternative energy forms.
Pickens gained national prominence in the 1980s with a series of takeover attempts, targeting Phillips Petroleum and other companies.
He's also given hundreds of millions to his alma mater, Oklahoma State University.
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