YouTube users will see the animated overlay, which can be clicked on to view the full ad, for a only few seconds once each video clip has started.
Trials showed that three quarters of those who clicked on the overlay watched the entire ad and only 10% chose to close the overlay.
The format, named InVideo, has been introduced by Google to maximise income from the site, which it bought late last year for $1.65bn (£830m).
The new service launches this week in the US. Google plans to launch the service in the UK but no launch date has yet been announced.
Eileen Naughton, Google's director of media platforms, said the use of InVideo advertising within clips was one of a number of formats looked into and was found to be the most appealing to users, many of whom are concerned about the over commercialisation of the site.
She said the trials also showed that users were between five- and 10-times more likely to click through to the full ad than with traditional online formats, such as banner advertising.
So far around 50 advertisers have signed up to the new format. One example of its use, shown to journalists at a media launch of the format, was a trailer for the film 'Hairspray', within a clip about hairstyling from the Ford modelling agency.
The new format will be initially restricted to content provided by key sponsors and professional or approved content providers.
Google's user survey found that 73% did not mind the overlay format as long as content remained free.
According to the latest figures from Nielsen/NetRatings, YouTube is the world's seventh most popular site in terms of unique users. In June this year the site had 94.5m unique users across the world, a 47% increase on December 2006, when it had 64.3m.