According to the results, released by the Audit Bureau of Circulation, Metro saw its traffic rise from 12.7 million in August to just under 16.8 million last month.
The new figure for monthly unique browsers marks a 177 per cent increase since January, the month after its redesign was unveiled. The site had suffered a 32 per cent fall in unique browsers in December.
Today's statistics for September also show a 33.4 per cent rise in average daily users, from 552,692 to 737,176.
Year-on-year the website has recorded an increase of 124.9 per cent in monthly browsers, and a 137 per cent rise in average daily traffic.
The chart below shows the journey of Metro's website traffic on a monthly basis since October 2012, as well as the breakdown in UK and 'rest of world' traffic. (Unfortunately there appears to be an error with the horizontal axis of the line charts below when viewed in Firefox - to see the correct data on the axis place your cursor over the variables in question).
In a statement, Dominic Rowell, chief customer officer for Metro said September's rise in traffic could be put down to "a team dedicated to delivering a great user-experience through both technology and content".
"The initial catalyst was the mobile-first redesign in December 2012 which has led to an audience increase of 177 per cent since January.
"During this period, mobile audience has increased by 225 per cent and social media referrals have gone up five-fold – which we attribute to the improved user experience as well as a focus and frequent promotion of socially-shareable content designed with mobile and our urban audience in mind."
The next highest year-on-year increase in monthly and daily browsers reported by ABC was for Mirror Group Digital, with a 103.6 per cent increase in daily browsers and 97.7 per cent rise in monthly traffic.
Returning to the month-on-month results, traffic to MailOnline continues to increase with 146 million browsers in September, a rise of 6.2 per cent on August.
The Independent and the Telegraph were the only other two audited titles to record growth in both daily and monthly traffic. The Independent recorded a 6.6 per cent rise in daily traffic, and 7.8 per cent rise in monthly traffic, to 115.3 million browsers, a new record for the site. The Telegraph followed with a 3 per cent increase for daily stats and 4.5 per cent for monthly results.
The Guardian (under its new global domain theguardian.com), recorded a 1.8 per cent rise in daily traffic but a slight decline in monthly browsers of 0.5 per cent. Mirror Group Digital average daily traffic fell 13.2 per cent, while its total traffic for September fell 9.6 per cent compared to August.
The graph below shows the recorded monthly traffic for the newspaper websites since October last year.
The chart below shows the UK and 'rest of the world' breakdown for website traffic across newspaper websites.
September also marked the introduction of Publication Active Viewers, which was included by the Times and Sunday Times in reference to their tablet editions.
The new metric looks at "the number of copies of a publication actively opened by an individual using a particular device", according to a release.
"ABC launched new Publication Active View metrics for Digital Publications earlier this year to allow publishers to measure digital usage more comprehensively and deliver increased accountability for this sector," the release adds.
Charts produced by Journalism.co.uk's technology editor Sarah Marshall, using Datawrapper