The New York Times introduced a plan on Thursday to begin charging the most frequent users of its Web site $15 for a four-week subscription in a bet that readers will pay for news they are accustomed to getting free.

Beginning March 28, visitors to NYTimes.com will be able to read 20 articles a month without paying, a limit that company executives said was intended to draw in subscription revenue from the most loyal readers while not driving away the casual visitors who make up the vast majority of the site’s traffic.

Once readers click on their 21st article, they will have the option of buying one of three digital news packages — $15 every four weeks for access to the Web site and a mobile phone app (or $195 for a full year), $20 for Web access and an iPad app ($260 a year) or $35 for an all-access plan ($455 a year). All subscribers who take home delivery of the paper will have free and unlimited access across all Times digital platforms except, for now, e-readers like the Amazon Kindle and the Barnes & Noble Nook. Subscribers to The International Herald Tribune, which is The Times’s global edition, will also have free digital access.

No American news organization as large as The Times has tried to put its content behind a pay wall after allowing unrestricted access. The move is being closely watched by anxious publishers, which have warily embraced the Web and struggled with how to turn online journalism into a profitable business.