Regulation of toll-like receptor induced signalling continues to attract interest due to the importance of this process in physiology and in the pathogenesis of several chronic diseases. The control of release of macrophage-derived inflammatory mediators in innate immunity is therefore of both therapeutic and scientific interest, but is not completely understood. We have recently undertaken a genome-wide functional screen in Raw 264.7 cells to identify novel regulators of inflammatory signalling in macrophages. Here we report characterisation of one of the hits from this screen, TMEM203. This is a conserved protein of unknown function and is a novel intracellular mediator of LPS induced TLR4 signalling. TMEM203 is an intracellular protein, which transiently co-localises with endosomal proteins and effector molecules involved in “late TLR4 signalling” events. siRNA knockdown of TMEM203 impaired TLR induced inflammatory cytokine activation, thus demonstrating a critical role for this molecule in inflammatory signalling.