Kennedy, 68, was taken to Tufts New England Medical Center about 3 a.m. Tuesday, said Rep. Patrick Kennedy, D-R.I.
"We're indebted to some anonymous pedestrian who found her and picked her up and got her help," he told the Boston Herald for its Wednesday editions.
Details of exactly what happened and how she ended up in the street were unclear. There was no police report on the incident. Joan Kennedy, who divides her time between Cape Cod and a Boston condominium, has struggled with alcoholism. She spent time in a number of rehabilitation programs following arrests for drunken driving.
The senator said Wednesday that he's proud of his children for their loyalty to his ex-wife.
"This has been a sad time," he said. "We're hoping for the best."
The couple had three children together — Kara Kennedy Allen, 45, Edward M. Kennedy, Jr., 44, and Patrick Kennedy, 37 — before divorcing in 1982.
The children recently took temporary guardianship of their mother to ensure she receives treatment for her alcoholism. Patrick Kennedy is seeking to become her permanent legal guardian.
Joan Kennedy worked as an advocate for the mentally retarded and for cancer research and was a supporter of the Pine Street Inn, a Boston-based homeless shelter, and Children's Hospital.
Patrick Kennedy announced Wednesday that he has decided not to run for Senate in 2006. His announcement did not mention his family responsibilities as a contributing reason for his decision.
"I would ... like to thank everyone for their outpouring of support for my mother," he said in a statement. "My family means everything to me, and I would appreciate you respecting my family's privacy at this time."