A Kennedy spokeswoman confirmed that the senator left Washington Hospital Center, where he had been resting overnight for observation.
The spokeswoman, who requested anonymity because the announcement was not official, said Kennedy was in good spirits and that doctors wanted him to get some rest.
Doctors on Tuesday had blamed fatigue for the seizure. Kennedy, who has been under treatment for a brain tumor since last year, suffered the seizure after attending the swearing-in of President Barack Obama.
He was talking with family and friends while he was at the hospital, according to a statement releasd by his office on Tuesday.
The 76-year-old Democrat was diagnosed last May with a particularly aggressive type of brain tumor and been treated with surgery, radiation and chemotherapy.
A doctor not connected with the senator's care, Dr. Matthew Ewend, neurosurgery chief at the University of North Carolina, said it is not unusual for patients recovering from brain tumors to suffer seizures.
Kennedy's seizure at the inaugural lunch was witnessed by fellow senators. Longtime friend Sen. Chris Dodd, a Democrat, , quoted Kennedy as saying, "I'll be OK, I'll see you later" as he was put into an ambulance on Tuesday.
Kennedy had appeared in good health and spirits a few hours earlier when he stepped out of the Capitol building and onto the inauguration platform where Obama took the oath of office. At the luncheon, Obama told lawmakers his prayers were with the stricken senator and his family.
Kennedy has pledged to forge a breakthrough on health care reform working with Obama and Democrats who control Congress. Kennedy has been a leading champion of health care reform for decades.