The West Midlands title will continue to publish "edited highlights" of the day's breaking news on the free-to-view front page of the site, but other content will no longer be freely accessible.
Deputy editor Keith Harrison told Journalism.co.uk that the new digital offering was a way of "rewarding our loyal readers".
The premium part of the site will not be sold to readers separately, but will be offered as part of a bundled print-and-online subscription.
The decision on what content will appear on which side of the paywall will be made on a case-by-case basis by the editors.
Harrison told Journalism.co.uk that the new site would launch in "a couple of months" and exact details of the charging structure were still being ironed out.
The experiment is limited to the Express & Star for the moment but the rest of the Midland News Association, such as the Shropshire Star, will be watching it closely.
"If it's a model that proves successful it will be rolled out across the group," Harrison said.
Speaking at Publishing Expo in London today, he added: "Other [regional publishers] have tried paywall content - some with varying results. I think the difference to what we're doing is we're not putting it behind the paywall as a standalone package.
"We're not trying to sell this as a standalone, in competition to the hard copy paper. It's a package to add value and help the overall circulation."
According to ABC, the Express & Star recorded an average daily print circulation of 116,992 in the second half of 2010
Expressandstar.com had 581,528 monthly unique users in the same period - a year-on-year increase of 15.5%.