The person spoke on condition of anonymity Monday night because neither team had announced the trade. It wasn't clear what compensation the 49ers would receive in return.
Haralson departed San Francisco team headquarters Monday night after saying his goodbyes before the team began a late practice, acknowledging that he had been traded while providing no further details other than, "I'm outta here."
Saints outside linebacker Smith will be sidelined for the season with a knee injury, he confirmed to ESPN's Ed Werder via text message. Butler is recovering from knee surgery that will sideline him most or all of the season.
Haralson said while walking to the parking lot, "I want to let them announce it," pointing upstairs to the team's front office.
He offered handshakes and hugs throughout the locker room, changed out of 49ers gear and into shorts and a T-shirt, then left the facility.
He told teammates he would fly out first thing in the morning to join his new team -- headed back to the Big Easy, where San Francisco lost the Super Bowl 34-31 to the Baltimore Ravens in February.
The NFC champion Niners have depth at linebacker, and Haralson was in a reserve role while beginning a comeback season after missing all of 2012's run with a torn left triceps.
In June, 49ers coach Jim Harbaugh acknowledged he wished he had used the return exemption in Haralson's situation, which would have allowed him to be activated and play again once healthy and not have to stay on season-ending injured reserve.
"It was really hard," Haralson said in a recent interview with The Associated Press. "I think that's hard for anybody, especially being in this business that we're in. That's what you get paid for, you get paid to come out and play football not sit back and rehab.
"It was hard on me, but it's one of those things you can't do anything about it. You take the bad and turn it into good and do as much as you can to stay around it and stay in it."
New Orleans' injured defense certainly will get Haralson up to speed in a hurry to play in Rob Ryan's new 3-4 scheme.
San Francisco faces New Orleans in Week 11 on Nov. 17 at the Superdome, and 49ers running back Frank Gore told Haralson, "When we play y'all, be easy on me."
Haralson, 29, was a 16-game starter on the 2011 defense that ranked No. 1 stopping the run.
A seven-year veteran, Haralson has 145 career tackles and 21½ sacks in 86 games. He had been set to be the primary backup last season behind Ahmad Brooks and Aldon Smith before he was injured in the third preseason game.
Haralson said he gained valuable expertise during his season on the sideline, and through helping his teammates in meetings, in practice and on game day.
"I never felt like I stepped away from football because I was still here throughout the whole injury and IR process," Haralson said. "It's one of the things getting back used to the football movements. As far as the mental part of it, I'll never forget that."
During the offseason, Haralson restructured his contract in a move to save the team salary-cap space. A talented run defender, Haralson enters his contract season with the chance to get more exposure as a starter for the Saints.
The Mercury News earlier reported the trade was with New Orleans.
Information from ESPN NFL Insider Ed Werder and The Associated Press was used in this report.