A stream providing full access to the first episode will be available tomorrow (Sept 6) until Sept 12 on the Yahoo TV web site. The episode will be aired without any ads.
ITV has acquired all rights to Supernaturalin the UK.
The show, which was regarded as one of the hottest properties at the LA Screenings in May revolves around two brothers combating supernatural forces and is slated for broadcast on ITV1 and ITV2.
The WB's move is only the second time a major US network has used the web to debut a new show - as part of a marketing drive.
The first episode will be available to watch anytime for the seven day period - but will not be downloadable.
Garth Ancier, chairman of The WB, told TV Week: "If we can get 100 million impressions from Yahoo!, that's a tremendous awareness builder for the show that we can't buy for almost any price in this society right now".
Last year The WB did a similar marketing move with its new drama Jack & Bobbythat had 700,000 visits over eight days. However the marketing move didn't ultimately help the show, which was still cancelled after its first series.