Transforming growth factor-β activated kinase-1 (TAK1) is a key regulatory molecule in toll-like receptor (TLR), interleukin-1 (IL-1), and tumor necrosis factor (TNF) signaling pathways. The activation of TAK1 is specifically regulated by two TAK1-binding proteins, TAB1 and TAB2. However, the roles of TAB1 and TAB2 in fish have not been reported to date. In the present study, TAB1 (CiTAB1) and TAB2 (CiTAB2) in grass carp (Ctenopharyngodon idella) were identified and characterized, and their expression profiles were analyzed after fish were infected with the pathogenic ciliate Ichthyophthirius multifiliis. The full-length CiTAB1 cDNA is 1949 bp long with an open reading frame (ORF) of 1497 bp that encodes a putative protein of 498 amino acids containing a typical PP2Cc domain. The full-length CiTAB2 cDNA is 2967 bp long and contains an ORF of 2178 bp encoding a putative protein of 725 amino acids. Protein structure analysis revealed that CiTAB2 consists of three main structural domains: an N-terminal CUE domain, a coiled-coil domain, and a C-terminal ZnF domain. Multiple sequence alignment showed that CiTAB1 and CiTAB2 share high sequence identity with other known TAB1 and TAB2 proteins, and several conserved phosphorylation sites and an O-GlcNAc site were deduced in CiTAB1. Phylogenetic tree analysis demonstrated that CiTAB1 and CiTAB2 have the closest evolutionary relationship with TAB1 and TAB2 of Danio rerio, respectively. CiTAB1 and CiTAB2 were both widely expressed in all examined tissues with the highest levels in the heart and liver, respectively. After infection with I. multifiliis, the expressions of CiTAB1 and CiTAB2 were both significantly up-regulated in all tested tissues at most time points, which indicates that these proteins may be involved in the host immune response against I. multifiliis infection.