De Mars Oosthaven is the larger of two harbours at Camping & Jachthaven De Mars, a family-run combination campsite and marina situated within the 400-hectare recreational area of Rhederlaag, near the village of Lathum in Gelderland.

The site lies in open connection with the IJssel. The Oosthaven sits on the east side of the campsite and serves as the main arrival point for visiting boaters, with the registration jetty for passers-by located at jetty A.

A harbour office is on site, along with a toilet block offering showers, washing machines, and dryers. Together with the smaller Westhaven on the west side, the total number of berths across both harbours reaches approximately 450.