The move comes ahead of a comprehensive redesign of Brand Republic, which will relaunch in the autumn, supplying content from 12 magazines including, Campaign, Marketing, PR Week, Eye and Media Week.
Smith will oversee the editorial development of the site as it expands its offering.
Smith has edited Media Week since May 2005 and was previously the editor of Revolution, the Haymarket-owned digital marketing title.
Gordon MacMillan's role as the editor of Brand Republic is unaffected by the appointment, although he will be developing the site's community dimension.
Rufus Olins, the managing director of Haymarket Brand Media, said: "Philip will be a terrific ambassador for Brand Republic and instrumental in removing the traditional boundaries between print and online media."