The Gunners had earlier on Saturday won deservedly 1-0 at Leicester to extend their lead, but City brought the advantage back to down to two points - albeit having played a game more - with their emphatic away win.
No City player has scored 27 goals in a single Premier League season. Sergio Aguero's 26 in 2014-15 was the previous best for the club before the prolific Haaland.
Julian Alvarez and Phil Foden added more for City, and an own goal by Chris Mepham wrapped up the visitors' scoring on an easy night on the south coast.
It is six matches without a clean sheet for Pep Guardiola's team, however, after Jefferson Lerma's 83rd-minute consolation.
Remarkably, Haaland has broken City's single-season scoring record in the league even though they still have 13 more matches to play.
"I'm very pleased with what I'm seeing every day," Guardiola said.
"The mood of the locker room is exceptional, the training sessions are really, really good.
"Today we came here and behaved in the way we behaved. It made me feel we still have the desire to try and to try. We will try to fight for this Premier League, I don't have any doubts."
Earlier, Gabriel Martinelli struck the only goal as Arsenal secured their win at Leicester's King Power Stadium.
The Gunners were the better side and won courtesy of Brazilian Martinelli's effort early in the second half.
Socceroo Harry Souttar suffered a second straight defeat for the Foxes and was lucky not to give away a penalty when he appeared to foul Bukayo Saka blatantly in the first half.
The second half was not a minute old when Leandro Trossard's pass through the legs of Souttar picked out Martinelli, who finished well from an acute angle.
"I think he was really good," Arsenal boss Mikel Arteta said of Trossard's afternoon.
"He's so good in tight spaces and small spaces with that creativity to open people up. I'm really happy we have him."
The visitors stemmed some late Leicester pressure to claim an 18th league win of the campaign and remain on course for a first title in 19 years.
Elsewhere, Junior Firpo scored the only goal for Leeds United as their new manager Javi Gracia launched his reign in style with a win over fellow strugglers Southampton.
The Elland Road club's first win since December 26 enabled them to climb out of the bottom three and left Southampton rooted to last place.
In-form Ollie Watkins scored for the fifth successive match in Aston Villa's 2-0 victory at Everton which dropped the Toffees back into the relegation zone.
Watkins' 63rd-minute penalty saw him become the club's first player to achieve the feat in the top flight since Paul Rideout in January 1985.
Substitute Emi Buendia made the game safe eight minutes from time to halt Villa's three-match losing run.
At London Stadium, Danny Ings scored twice on his first start for West Ham United as they climbed out of the relegation zone with a 4-0 win over Nottingham Forest.
Declan Rice added a curling third and substitute Michail Antonio rounded off a much-needed win for Hammers' manager David Moyes.
And in the late game at Selhurst Park, Liverpool moved above Brighton to seventh despite being held to a 0-0 draw by Crystal Palace.
With AP & Reuters.
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