The 7-foot-1, 325-pound center in the twilight of a brilliant career is coming to the Eastern Conference champions, hoping to add to his four NBA titles.
O'Neal signed a two-year contract worth about $3 million on Wednesday, a person with knowledge of the negotiations said on condition of anonymity because no details were released.
So this is where Shaq's colorful career will end after 20 seasons if he completes his contract with the Celtics.
One of the NBA's most charismatic players will finish with a legendary franchise on the parquet floor beneath 17 championship banners and the retired numbers of Bill Russell, Larry Bird, John Havlicek and other NBA greats.
After receiving little interest from other teams, O'Neal will play on that floor with Paul Pierce, Kevin Garnett and Ray Allen, Boston's Big Three since Garnett and Allen arrived before the 2007-08 season
From the first pick in the 1992 draft out of Louisiana State to dominant, rim-rattling inside force to complementary player, O'Neal's NBA travels have taken him to six teams and 15 All-Star games. He is the fifth-leading scorer with 28,255 points and 14th in rebounds with 12,921 in league history. He's averaged 24.1 points, 11.0 rebounds and 2.3 blocks and was the NBA MVP in 2000.
His foul shooting might still be a problem and his lumbering style could slow down a running game, but the Celtics need the big center because Kendrick Perkins is out, possibly until February, following knee surgery. Perkins was hurt in Game 6 of the NBA finals against Los Angeles and missed the Lakers' 83-79 win in Game 7.
"If we get to another Game 7 down the line, I'm sure we will be glad we have Shaq on the team," Celtics co-owner Wyc Grousbeck said.
The team considered too old by some, despite nearly winning the title, has decided to rely for two more years on a group of 30-plus stars, all likely headed for the Hall of Fame.
O'Neal, 38, joins Pierce, 32, Garnett, 34, and Allen, 35. The Celtics also signed free agent center Jermaine O'Neal, 31 -- a six-time All-Star -- after the season.
All five are signed for two seasons, except Pierce, who has four years left on the deal he signed after the season. Point guard Rajon Rondo, 24, also is in the fold for four more years.
"I am honored to be joining the Celtics," Shaq said in a statement issued by the club. "I have played against Paul, Ray, Kevin, Rajon, and Jermaine for many years and it will be great to be able to call them my teammates. I cannot wait to get to Boston to get started in pursuit of another championship."
The Celtics won their NBA-high 17th championship in 2008.
"It is not every day that you can add a player of Shaquille's caliber to your team," Celtics president of basketball operations Danny Ainge said in the statement. "His past experience speaks for itself and we believe that he is a great fit for our roster."
Not far from where the Celtics play, another large figure on the Boston sports scene spread the news.
"Hey, they signed Shaq here," David "Big Papi" Ortiz said out loud in the Red Sox clubhouse after taking batting practice at Fenway Park before the game with the Cleveland Indians.
Just which of the two O'Neals will start at center remains uncertain with Perkins recovering from surgery for torn ligaments in his right knee.
"I am very excited," Celtics coach Doc Rivers said. "Adding a player like Shaq is a major plus and it fills a void for our team. He's a real nice piece of the puzzle and he will complement us in where we want to go this coming season."
O'Neal has won three championships with the Lakers along with his latest, in 2006, with Miami when he was teammates with Dwyane Wade.
The Cleveland Cavaliers signed O'Neal before last season, hoping he could help LeBron James win his first title. O'Neal averaged a career-low 12.0 points, 6.7 rebounds and 1.2 blocks in 53 games, and the Cavaliers lost the Eastern Conference semifinals to the Celtics in six games.
In his last game, O'Neal had 11 points and 4 rebounds in a 94-85 loss in Boston.
On Tuesday, O'Neal sounded unsure if he would ever play another NBA game after a difficult offseason of uncertainty.
"For me, it's been a real thinking process," he said in Orlando. "I came into the league very graciously and want to go out very graciously. My main thought was I would like to play for a winning franchise, somebody that's used to winning, somebody that keeps winning. Hopefully, I'll make my decision here in the next one or two days."
Now it's been made. He will wear No. 36 in Boston.
Information from The Associated Press was used in this report.