That's according to U.S. Supreme Court spokeswoman Kathy Arberg, who says the Chief Justice had been running errands and was stepping off his boat onto a dock when he experienced the seizure.
Arberg says Roberts, 52, was taken by ambulance to the Penobscot Bay Medical Center, where he underwent a "thorough neurological evaluation, which revealed no cause for concern."
Roberts is fully recovered from the incident, says Arberg, adding that Roberts experienced a similar medical problem in 1993. CBS News chief White House correspondent Jim Axelrod reports that episode, which occurred while Roberts was playing golf, did not come up during his confirmation hearings because it was thought to be an isolated event.
Monday's incident occurred around 2 p.m. on a dock near the home in Port Clyde on Maine's Hupper Island. Port Clyde, which is part of the town of St. George, is about 90 miles by car northeast of Portland, midway up the coast of Maine.
Roberts was taken by private boat to the mainland and then transferred to an ambulance, St. George Fire Chief Tim Polky said.
"He was conscious and alert when they put him in the rescue (vehicle)," Polky said. The hospital, in Rockport, did not immediately return a call from The Associated Press.
Named to the court by President Bush in 2005, Roberts is the youngest justice on a court in which the senior member, John Paul Stevens, is 87. Bush was informed of the hospitalization by his chief of staff, Josh Bolten, the White House said.
Roberts is the father of two young children.
Doctors called Monday's incident "a benign idiopathic seizure," Arberg said. The White House described the January 1993 episode as an "isolated, idiosyncratic seizure."
Larry Robbins, a Washington attorney who worked with Roberts at the Justice Department in 1993, said he drove Roberts to work for several months after the incident. Robbins said Roberts never mentioned what the problem was and he never heard of it happening again.
In 2001, Roberts described his health as "excellent," according to Senate Judiciary Committee records.
Roberts became chief justice after the death of William Rehnquist in September 2005, although Bush had first chosen him to take Sandra Day O'Connor's seat when she announced her retirement earlier that year.
He had served as an appellate judge in Washington and spent more than a decade before that as a lawyer at the Hogan and Hartson law firm, where he specialized in arguing cases before the Supreme Court.
Roberts also served in the Reagan and Bush administrations in the 1980s and '90s. He was a clerk for Rehnquist after graduating from Harvard Law School.
Roberts spent a couple of weeks in Europe in July, teaching a course in Vienna and attending a conference in Paris. He was at the court in Washington late last week.