The 47-year-old Illinois Democrat and son of Jesse Jackson, the leading civil rights leader, has not been on Capitol Hill since late May.
Earlier in July -- one month after announcing he was on a leave of absence due to a "medical condition" -- the congressman's office released a statement saying he was "receiving intensive medical treatment at a residential treatment facility for a mood disorder."
"Congressman Jesse Jackson Jr. Has arrived at Mayo Clinic in Rochester, Minn., for extensive inpatient evaluation for depression and gastrointestinal issues," a short family statement released by the clinic in Minnesota said.
"Congressman Jackson and his family are grateful for the outpouring of support and prayers that have been received throughout his care," it said.
The name of the hospital was not announced at the time. But Friday's statement makes clear that Jackson is being treated at the Mayo Clinic, one of America's and world's most renowned hospitals.
"Further information will be released as Congressman Jackson's evaluation proceeds," the statement said.
Jackson is the US House of Representative for Illinois's 2nd congressional district, serving since the special election in 1995.
Jackson's illness comes as the House Ethics Committee is examining allegations that in 2008 he or or one of his associates offered to raise funds for then-Illinois Governor Rod Blagojevich in exchange for Jackson being appointed to the Senate seat vacated by Barack Obama after he was elected US president, CNN reported.
The congressman has maintained his innocence and pledged to continue to cooperate with authorities.
Blagojevich was convicted last year on corruption charges in connection with his efforts to profit from appointing the successor to the Senate seat. He began serving a 14-year sentence in March.
