High TRABD expression is associated with tau pathology in patients with Alzheimer’s disease; however, the function of TRABD is unknown. Human TRABD encodes a mitochondrial outer-membrane protein. The loss of TRABD resulted in mitochondrial fragmentation, and TRABD overexpression led to mitochondrial clustering and fusion. The C-terminal tail of the TRABD anchored to the mitochondrial outer membrane and the TraB domain could form homocomplexes. Additionally, TRABD forms complexes with MFN2, MIGA2, and PLD6 to facilitate mitochondrial fusion. Flies lacking dTRABD are viable and have normal lifespans. However, aging flies exhibit reduced climbing ability and abnormal mitochondrial morphology in their muscles. The expression of dTRABD is increased in aged flies. dTRABD overexpression leads to neurodegeneration and enhances tau toxicity in fly eyes. The overexpression of dTRABD also increased reactive oxygen species (ROS), ATP production, and protein turnover in the mitochondria. This study suggested that TRABD-induced mitochondrial malfunctions contribute to age-related neurodegeneration. Display omitted •TRABD is a mitochondrial outer-membrane protein•TRABD modulates mitochondrial homeostasis and function•The self-interaction of TraB domain in the TRABD promotes the tethering of mitochondria•Elevated TRABD levels exacerbate neurodegeneration and tau toxicity in fly eyes Zhou et al. show that TRABD, a protein associated with Alzheimer’s disease, is a highly conserved mitochondrial outer-membrane protein whose expression increases with age. TRABD modulates mitochondrial morphology by promoting mitochondrial tethering and homeostasis. Elevated TRABD levels exacerbate neurodegeneration and tau toxicity in fly eyes.