Philippe Coutinho scored a stunning winner at Camp Nou, but the occasion was all about Iniesta, who made his 674th and final appearance for Barcelona, 22 years after initially joining the club.
The 34-year-old, who has won 32 trophies in 16 years as part of the first team, was afforded a standing ovation and an emotional farewell when he was taken off with eight minutes to spare.
Barcelona rarely showed any signs of letting their one-goal lead slip after that, with their victory ensuring that they finish the season 14 points clear of second-placed Atletico Madrid.
All eyes were on Iniesta at kickoff, but Sociedad threatened to spoil the party early on when Adnan Januzaj's cross into the box picked out Willian Jose, who could only plant his header straight at Marc Andre ter Stegen.
Indeed, it was the visitors who continued to look the most likely to score in the opening 10 minutes, with Ter Stegen forced into another save to deny Januzaj before Willian Jose drilled a powerful strike narrowly wide of the target.
Even some Sociedad fans may have been secretly wanting to see an Iniesta goal, though, and the legendary Spanish midfielder had his first sighter after 10 minutes when he turned away from his marker before rippling the side-netting with a low strike from the edge of the box.
There was a better chance for another of Barca's stalwarts in the 18th minute when Gerard Pique was left unmarked from a corner, but he scuffed his attempted volley into the grateful arms of the keeper.
After a slow start to the match, the champions were suddenly in control and they squandered another clear opening in the 25th minute when Ivan Rakitic met Ousmane Dembele's cross with a firm header, only to steer it wide of the target.
Luis Suarez was the next to come close for the home side when he latched on to another cross from Dembele, getting in front of his marker but dragging his effort wide.
Sociedad's forays into the Barcelona half were increasingly rare, but they did create two more chances in the final 15 minutes of the half when first Mikel Oyarzabal turned and fired wide from inside the box and then Jose blazed over the top when he should have done better.
The visitors were fortunate to end the half with 11 men still on the field, though, as Raul Navas caught Dembele with a nasty challenge on his ankle three minutes before the interval, eventually forcing the Frenchman off after he tried to continue in the opening stages of the second half.
Just three minutes after Dembele's withdrawal Barcelona took the lead, and it was a goal worthy of all the headlines on any other night aside from Iniesta's farewell, as Coutinho - considered by some to be the Spaniard's heir apparent at Camp Nou - dipped inside before curling a stunning effort in off the far post which left Miquel Moya rooted to the spot.
Things looked like going from bad to worse for the visitors 10 minutes later when Lionel Messi was brought off the bench for one final cameo alongside Iniesta, and the Argentine seemed determined to make an impact during his short time on the field.
Pique first squandered another good chance when he directed his volley straight at the keeper, before Messi burst through the middle of the pitch to set up Denis Suarez, who seemed to be caught in two minds with his finish.
Messi then exchanged passes with Luis Suarez to create a chance for himself, hanging in the air at the back post to meet the lofted return ball with a header which needed to be saved by the keeper.
It looked far more likely that Barca would get the game's second goal if there was one, but Sociedad did fire one last warning shot in the 81st minute when Asier Illarramendi was afforded space to shoot from 25 yards, drilling a low strike narrowly past the far post.
There was a special moment immediately after that, though, as Iniesta was taken off to a standing ovation from Camp Nou, bidding farewell to all of his teammates on his way off the field as his illustrious career at the club came to an end.
The playmaker's replacement Paco Alcacer almost made sure of the points with five minutes remaining when he was played through on goal by Rakitic, but his first touch let him down and he blasted his subsequent effort off target.
There was also a notable farewell for the visitors as retiring captain Xabi Prieto was handed a late cameo on his final game for the club, but there was no doubt that the star of the show was Iniesta.
BARCELONA (4-3-3): Ter Stegen; Semedo, Pique, Mina, Alba; Rakitic, Busquets, Iniesta (Alcacer 82'); Dembele (D Suarez 53'), L Suarez, Coutinho (Messi 67')
REAL SOCIEDAD (4-2-3-1): Moya; Elustondo (Odriozola 71'), Llorente, Navas, De La Bella; Illarramendi, Zurtuza (Prieto 89'), Januzaj (Canales 63'), Juanmi, Oyarzabal; Willian Jose