With just 12 minutes left, Guardiola’s troubled side were clinging onto the lead given to them by Erling Haaland in the first half.
But they collapsed in the closing stages as Joao Pedro equalised before Matt O’Riley bagged Brighton’s winner seven minutes from full-time.
For the first time in his glittering career as a manager with City, Bayern Munich and Barcelona, Guardiola has been beaten four times in a row.
It was also the first time City had lost four successive games since a run between April and August 2006 under Stuart Pearce.
After exiting the League Cup to Tottenham and suffering a first league loss since December at Bournemouth, City were thrashed 4-1 by Sporting Lisbon in the Champions League in midweek.
The slump continued as they failed to kill off Brighton before the second half implosion that threatens to derail their bid for a fifth successive Premier League title.
Second-placed City are two points behind leaders Liverpool, who can extend that gap to five points if they beat Aston Villa at Anfield later on Saturday.
City have been beset by injury problems, most notably the loss of Ballon d’Or winner Rodri for the rest of the season.
Kevin De Bruyne, Nathan Ake and Manuel Akanji were only fit enough for the bench, while Ruben Dias, John Stones, Jeremy Doku and Jack Grealish were absent from the squad.
Guardiola had insisted he would solve City’s problems but he has so far been unable to come up with the answer to their mounting crisis.
City had paid for failing to turn their dominance into more than a one-goal lead against Sporting and profligacy was a problem again on the south coast of England.
City in turmoil
Savinho should have put City ahead when Mateo Kovacic’s pass sent him surging into the Albion area, but the Brazilian’s weak finish allowed Bart Verbruggen to save.
Kovacic was the catalyst when City took the lead in the 23rd minute with a goal equal parts inspiration and perspiration.
The Croatia midfielder’s inch-perfect pass split the Brighton defence but Haaland still had work to do as his initial shot was blocked by Verbruggen, leaving the Norwegian in a race with Jan Paul van Hecke to poke the ball in before it could be cleared.
Haaland’s 15th goal in all competitions this season was only the second time he had netted in his last six league appearances.
Haaland’s 75th league goal since joining City in 2022 was almost followed by the 76th moments later, his low drive repelled by Verbruggen at the near post.
Kovacic’s long-range volley forced another good stop from the increasingly over-worked Verbruggen.
But City fatally dropped their intensity just before half time, allowing Fabian Hurzeler’s side to gain a foothold.
Jack Hinshelwood wasted a golden opportunity to haul Brighton level early in the second half as his close-range header from Pervis Estupinan’s cross was palmed away by Ederson.
Ederson raced off his line to save at Kaoru Mitoma’s feet before Pedro scuffed wide with the goal at his mercy.
Pedro made amends for that miss as the Brazilian punished woeful City defending to equalise with 12 minutes left.
Mitoma was given too much space for a pass into Welbeck in the six-yard box.
Rico Lewis’s block-tackle stopped Welbeck shooting but Josko Gvardiol and Matheus Nunes didn’t clear the danger, allowing Pedro to pounce with a close-range strike.
Guardiola slumped into his seat but there was worse to come for the City boss in the 83rd minute.
Pedro slipped his pass through City’s leaky defence and O’Riley timed his run perfectly to clip a composed finish past Ederson from 10 yards to leave the champions in turmoil.