Zymoseptoria tritici is an important foliar pathogen of Triticum aestivum (cultivated wheat). Control of Septoria tritici blotch disease in the field, caused by Z. tritici, is mainly facilitated by fungicidal chemical applications and planting of wheat breeds containing a combination of Septoria resistance loci (STB loci). Currently, molecular mechanisms underlying STB trait resistance are not fully understood and efficacy and availability of chemical fungicides is limited. In order to improve control of this disease in the field requires greater understanding of the processes underlying successful wheat infection. Secreted "effector" proteins enable plant pathogenic fungi to manipulate host defences for successful infection. Characterisation of effector proteins, demonstrating an important function during wheat infection may provide a novel approach to developing new disease control strategies, as well as aiding in our understanding of disease dynamics. Secreted Lysin Motif-containing (LysM) proteins are now well-known effector molecules deployed by fungal pathogens, as originally defined for the Extracellular protein 6 (Ecp6) gene in Cladosporium fulvum. Using homology and in silico analysis three putative LysM containing secreted effector genes were identified in the Zymoseptoria tritici genome. Gene sequencing analysis confirmed they are highly conserved in multiple Z. tritici isolates. Expression analysis indicates two of these genes, Mycosphaerella graminicola 1LysM (Mg1L) and Mycosphaerella graminicola 3LysM (Mg3L), are highly expressed during symptomless wheat infection. Collaborative protein functional analysis indicates a role in chitin-binding and protection against plant defence compounds for Mg1L and Mg3L. Mutational analysis indicates that Mg3L in particular, plays an essential role during Z. tritici wheat infection. This represents the first fully characterised effector protein in Z. tritici and emerges alongside functional characterisation of LysM effector proteins in other plant pathogenic species e.g. Magnaporthe oryzae Secreted LysM Proteinl (Slp1). Mg3L protein orthologues are found in many plant pathogenic ascomycete fungal genomes and two plant pathogenic oomycete genomes.