Myeloid differentiation factor 88 (MyD88) is an indispensable component of the toll-like receptor (TLR)- and interleukin 1 receptor-mediated signaling pathways and plays an important role in defensive immune responses. In this study, we cloned MyD88 from the Nile tilapia, Oreochromis niloticus (OnMyD88). Phylogenetic analysis showed that OnMyD88 is grouped with other teleost MyD88s and has the highest amino acid identity and similarity to fugu MyD88. The gene organization of OnMyD88 is similar to MyD88 in several other fish species, except for Atlantic salmon. Gene expression studies revealed that OnMyD88 is highly expressed in immune-related tissues, such as spleen and head kidney, and is up-regulated in spleen and head kidney tissues from fish infected by Streptococcus agalactiae. Functional studies showed that overexpressed OnMyD88 could activate NF-κB and AP-1 in human embryonic kidney (HEK) 293 and tilapia head kidney (THK) cell lines and induce the expression of proinflammatory cytokines such as IL-1β and IL-12b and chemokine IL-8 in THK cells. The result of the co-immunoprecipitation demonstrated that OnMyD88 can be recruited by OnTLR25 as its adaptor. Taken together, our study indicates that OnMyD88 plays an essential role in the innate immune responses of Nile tilapia against bacterial infection.