A week after a coalition of big public relations firms vowed not to secretly edit their clients' Wikipedia pages, the foundation that runs the online encyclopedia is going a step further by strengthening its rules against the practice.

Beginning Monday, changes in Wikipedia's terms of use will require anyone paid to edit articles to disclose that arrangement. Katherine Maher, the nonprofit Wikimedia Foundation's chief communications officer, said the changes address a sentiment among volunteer editors that, "we're not an advertising...