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The γ-aminobutyric acid receptors (GABARs) mediate fast inhibitory transmission in central nervous system of insects and are important targets of insecticides. An auxiliary subunit, Shisa7, was identified in mammals as a single-passing transmembrane protein. However, the homology gene(s) of Shisa in invertebrates has not been reported to date. In the present study, a homolog Shisa gene was identified from the two-spotted spider mite, Tetranychus urticae Koch. Its open reading frame had 927 base pairs and encoded 308 amino acid residues, which has a typical Shisa domain at 13th-181st amino acid residues. According to the phylogenetic tree, the invertebrate Shisa was categorized apart with those of vertebrate, and Tu Shisa showed closest relationship with the Shisa9 of velvet mite, Dinothrombium tinctorium (L.). In the electrophysiological assay with two-electrode voltage clamp, the GABA-activated Tu RDL channel was functionally formed in the Africa clawed frog Xenopus laevis (Daudin) oocytes (EC 50 = 53.34 μM). No GABA-activated current could be observed in Tu Shisa-expressed oocytes, whereas Tu Shisa could reduce the sensitivity of TuRDL / TuShisa (mass ratio of 1: 4) channel to GABA. The homology structural models of Tu RDL and Tu Shisa were built by the SWISS-MODEL server, their interaction was predicted using Z -DOCK and three predicted hydrogen bonds and interface residues were analysed by PyMOL. Meanwhile, the key interface residues of Tu Shisa affected the stability of complex were calculated by Discovery Studio 2019. In conclusion, the Tu Shisa, as the first reported invertebrate Shisa, was explored and functionally examined as the GABARs auxiliary subunit. Our findings provide a basis for research of invertebrate Shisa. [Display omitted] • Tu Shisa was first isolated from two-spotted spider mite Tetranychus urticae Koch. • Tu Shisa could reduce the sensitivity of Tu RDL channel expressed in Xenopus laevis oocytes to GABA. • Tu Shisa interacted with Tu RDL by three hydrogen bonds. • GLY29 and GLY18 of Tu Shisa were two key interface residues affected the stability of Tu RDL- Tu Shisa complex. [ABSTRACT FROM AUTHOR]