Marnitz joins the Dragons from South African side Blue Bulls where he took on the role of backs and defence coach and most recently head coach.
Prior to his time coaching within the Blue Bulls development programme, the experienced coach took over as head coach of the All Ireland league side Buccaneers for four seasons.
The Johannesburg born scrum-half linked up with the Dragons during pre-season preparations at the beginning of August to look after the defence and exit strategy areas.
Commenting on the announcement, Marnitz said: “I am relishing the opportunity to work in Welsh Rugby.
“There are a lot of great rugby traditions within the region, and my first impressions are that we have a young exciting group of players who are hungry to learn and are hard working.
“I am also looking forward to putting our systems in place and seeing how the group evolves over the next couple of weeks.”
Dragons’ Head Coach, Bernard Jackman added: “Hendre is a coach that I got to know well when he coached in Ireland in 2011. He returned to South Africa to play a very important role in the Bulls elite development programme.
“He has coached and helped to produce many Springbok Internationals and his knowledge and expertise will be invaluable, in bringing through the young Dragons that we believe will bring success to our region and who will represent Wales too."