Ruddock, who led Wales to Grand Slam glory in 2005, will lead out Marshall's stars to face Warren Gatland's team who conquered the Six Nations this year.
Ruddock - who has been asked to Williams to do the honours in the clash - has been given clearance by Warriors to take charge for the day and admitted he was excited about returning to Wales for the fixture.
"I am looking forward to working with the World XV against a very strong Wales team," he said. "Particularly as one of the greatest wingers ever to play for Wales, Shane Williams, has asked me personally to coach the team as part of his testimonial.
"Justin Marshall will captain the World XV and I'm delighted to be able to work with him again having toured with him to South Africa a few years ago.
"Shane Williams is one of the nicest guys in world rugby as well as being one of the most exciting players to watch. I always enjoyed working with Shane both at Wales A and senior level and I'm pleased to be able to help out in such a small way to assist his testimonial."