They will be held on a "challenging 75km single-lap course".
It is part of the "ever popular" Selkirk Mountain Bike Marathon open cycling event on 2 May.
A British Cycling spokesman said Selkirk had "rapidly become the home" of the championships and had a "fine reputation for the excellence of its trails" as well as its infrastructure.
The British Mountain Bike Marathon Championships are returning to the Borders town of Selkirk in May.