Mr. Ford, 92, was admitted Saturday to Eisenhower Medical Center near his home in Rancho Mirage in Southern California, the same facility where he was briefly hospitalized a month ago.
"He is responding to treatment and doing well," chief of staff Penny Circle said in a brief statement.
Mr. Ford was expected to be released Thursday if his condition continues to improve, Circle said.
In mid-December, Mr. Ford underwent routine tests at Eisenhower and was hospitalized overnight because of what Circle called "a horrible cold." It wasn't clear if the cold led to the bout with pneumonia, she said.
Based on his age, it was felt that he should receive intravenous antibiotics at the hospital, Circle said Monday.
Hospital spokeswoman Elizabeth Wholihan referred inquiries to Circle, who said no further updates on Mr. Ford's condition would be made until Wednesday.
In 2000, Mr. Ford suffered two small strokes while attending the Republican National Convention in Philadelphia and spent about a week in the hospital. Doctors said there was no apparent brain damage. Three years later, he was hospitalized for a night for dizziness.
Mr. Ford became the nation's oldest living former president with the death of Ronald Reagan in June 2004. Reagan was 93.