Reid, a Nevada Democrat, made the announcement on the Senate floor, Politico reported.
"He is at Walter Reed, not for a checkup," Reid said. "He is there because he is infirm. He is sick."
A Dole aide told NBC News he is "doing very well" and will be discharged Wednesday.
Dole, 89, was the Republican presidential nominee in 1996 and was President Gerald Ford's running mate in 1976. He represented Kansas in the U.S. Senate from 1969 to 1996 and served in the U.S. House of Representatives from 1961 to 1969.
He was Senate majority leader from January 1995 until June 1996.
Reid told Senate colleagues Tuesday Dole called him a few days ago to let him know he was at Walter Reed, ABC News said.
"He is such a stalwart figure in the history of America," Reid said. "He had all the qualities of a leader that I admire and certainly would wish I had."
Dole was hospitalized in January 2011 for treatment of a minor infection, two months after he had been discharged following a 10-month stay at Walter Reed while he recovered from pneumonia following knee surgery. He was hospitalized in 2009 for multiple problems related to a skin graft procedure.
He was hospitalized at Walter Reed for 40 days after doctors determined he had internal cranial bleeding following hip replacement surgery in December 2004.
He received two Purple Hearts and the Bronze Star for his service in the Army during World War II.