Richards, 72, went in for tests Monday and got the diagnosis Tuesday, said spokesman Bill Maddox.
Maddox said the former governor is waiting to hear from M.D. Anderson about how advanced the cancer is and what her chances are.
"She's facing this challenge," Maddox said.
Richards, a Democrat, was governor from 1991 to 1995, losing a re-election bid to George W. Bush. Since 2001, she has been an adviser at a public relations and lobbying firm.
According to the American Cancer Society, more than 13,000 people in the United States are diagnosed with esophageal cancer each year, usually when the disease is at a late stage. The five-year survival rate is 16 percent for white patients and 9 percent for blacks, the organization said.
Maddox said he did not know if Richards had a specific problem that sent her to the doctor or if the cancer was discovered during a routine visit.