The Guardian, which unveiled its new size on 12 September, reported a 7 per cent year-on-year sales increase to 404,187 for September, according to the Audit Bureau of Circulations.
Paul Thomas, the managing partner at MindShare, said: "While I think it's great that it has picked up a sales increase, the proof of the pudding will come over a longer term than three weeks."
Agencies also said that The Guardian had made heavy use of promotion since its relaunch, including a free DVD of The Madness of King George.
The first full-month ABC figure for the Berliner format will cover October.
Carolyn McCall, the chief executive of Guardian Newspapers Limited, said: "We look forward to the October ABC with confidence."
Other quality titles had a good month. The Times sale rose by close to 6 per cent to 699,425, while The Daily Telegraph and The Independent were steady on 904,283 and 262,552 respectively.
The Daily Mail's sale was down 2.8 per cent year on year to 2,373,756 but the Daily Express was hit by a larger fall of 13 per cent to 831,373. Sales among the red-tops continued to fall, with The Sun down 1 per cent to 3,300,208 and the Daily Mirror down 2.9 per cent to 1,741,740.