Justice John Paul Stevens, the senior member, extended to the 50-year-old Roberts wishes for "a long and happy career in our common calling."
Roberts wore a plain black robe, without the gold arm stripes that had been used by his predecessor, William H. Rehnquist. Rehnquist died last month at age 80 after 33 years on the high court, 19 of them as chief justice.
President Bush and Roberts' family watched as Roberts repeated the oath he had taken at the White House on Thursday. The events briefly delayed argument sessions, on gasoline taxes and worker pay cases, for the opening day of the court's 2005-06 term.
In choosing Roberts, Mr. Bush passed over Justice Antonin Scalia, a conservative who had been considered a prospect for the job. Scalia was out of town last week and was the only justice absent from Roberts' swearing-in ceremony at the White House.
President Bush was among those invited to the ceremony, which comes shortly after his announcement that he has to replace retiring Justice Sandra Day O'Connor.
"The new Chief Justice earns his wings immediately," says CBSNews.com Legal Analyst Andrew Cohen. "The Oregon assisted suicide case is one of the biggest of the term and it comes at him on his third day on the bench. No time to ease into the role before the controversy begins."
Roberts is the court's newest and youngest member, but also its leader.
"It will take some time to figure out what the dynamics of the court are," said Andrew Koppelman, a law professor at Northwestern University. "It will take them awhile to get to know each other."
Complicating matters for the term is the uncertain status of O'Connor, who had expected to be off the court by now so she would have more time to care for her ill husband. She announced her retirement July 1, but the chief justice's death delayed the plans.
Her retirement starts the day her successor is confirmed, which is expected to take at least two months, if not longer if the president's pick is contested.
O'Connor, a 75-year-old moderate and key swing voter, will continue participating in cases. Because rulings take months to prepare, her votes would not count if she retires before they are done.
"O'Connor could be on the court all term and end up casting deciding votes," Koppelman said.
The Supreme Court meets for nine months. Its first week will be shorter than usual.
On Monday, the justices were hearing two cases, one that asks if companies must pay for workers' time spent changing into protective gear and safety uniforms, and a second that questions whether states may impose taxes on fuel that is sold on Indian reservations.
Justices were not meeting on Tuesday because of the Jewish holiday Rosh Hashanah.
Another case Wednesday will clarify how parents of disabled children can contest education services.
Some significant cases also will be argued later this year, including a review of a parental notification law from New Hampshire and an appeal involving a claim that an anti-abortion group's protests violated federal racketeering laws.
Another high-profile case asks if the government can withhold federal funds from colleges that bar military recruiters in protest of the Pentagon's "don't ask, don't tell" policy on homosexuality.