The 11 members of the Membrane-associated RING-CH (MARCH) ubiquitin ligase family are relatively unexplored. Upon exogenous (over)expression, a number of these ligases can affect the trafficking of membrane molecules, but only in case of MARCH-1 endogenous functions have been demonstrated. Of the other endogenous MARCH proteins, no functions or substrates are known. We report here that TRAIL-receptor (R)1 is a physiological substrate of the endogenous MARCH-8 ligase. The two human TRAIL death receptors play a role in immunosurveillance and are targets for cancer therapy, since they selectively induce apoptosis in tumor cells. We demonstrate that TRAIL-R1 is downregulated from the cell surface, with great preference over TRAIL-R2, by exogenous expression of MARCH ligases that are implicated in endosomal trafficking, such as MARCH-1 and -8. MARCH-8 attenuated TRAIL-R1 cell surface expression and apoptosis signaling by virtue of its ligase activity. This suggested that ubiquitination of TRAIL-R1 was instrumental in its downregulation by MARCH-8. Indeed, in cells with endogenous MARCH expression, TRAIL-R1 was ubiquitinated at steady-state, with the conserved membrane-proximal lysine 273 as interaction and potential acceptor site. This residue was also essential for the interaction of TRAIL-R1 with MARCH-1 and MARCH-8 and its downregulation by these ligases. Gene silencing identified MARCH-8 as the endogenous ligase that ubiquitinates TRAIL-R1 and attenuates its cell surface expression. These findings reveal that endogenous MARCH-8 regulates the steady-state cell surface expression of TRAIL-R1.