Alzheimer's disease and related disorders feature neurofibrillary tangles and other neuropathological lesions composed of detergent-insoluble tau protein. In recent structural biology studies of tau proteinopathy, aggregated tau forms a distinct set of conformational variants specific to the different types of tauopathy disorders. However, the constituents driving the formation of distinct pathological tau conformations on pathway to tau-mediated neurodegeneration remain unknown. Previous work demonstrated RNA can serve as a driver of tau aggregation, and RNA associates with tau containing lesions, but tools for evaluating tau/RNA interactions remain limited. Here we employ molecular interaction studies to measure the impact of tau/RNA binding on tau microtubule binding and aggregation. To investigate the importance of tau/RNA complexes (TRCs) in neurodegenerative disease, we raised a monoclonal antibody (mAb TRC35) against aggregated tau/RNA complexes. Here we show native tau binds RNA with high affinity but low specificity, and tau binding to RNA competes with tau mediated microtubule assembly functions. Tau/RNA interaction in vitro promotes the formation of higher molecular weight tau/RNA complexes which represent an oligomeric tau species. Co-expression of tau and poly(A)45 RNA transgenes in Caenorhabditis elegans exacerbates tau related phenotypes including neuronal dysfunction and pathological tau accumulation. TRC35 exhibits specificity for Alzheimer's disease-derived detergent insoluble tau relative to soluble recombinant tau. Immunostaining with TRC35 labels a wide variety of pathological tau lesions in animal models of tauopathy, which are reduced in mice lacking the RNA binding protein MSUT2. TRC-positive lesions are evident in many human tauopathies including Alzheimer's disease, progressive supranuclear palsy, corticobasal degeneration, and Pick's disease. We also identify ocular pharyngeal muscular dystrophy as a novel tauopathy disorder where loss of function in the poly(A) RNA binding protein (PABPN1) causes accumulation of pathological tau in tissue from postmortem human brain. Tau/RNA binding drives tau conformational change and aggregation inhibiting tau mediated microtubule assembly. Our findings implicate cellular tau/RNA interactions as modulators of both normal tau function and pathological tau toxicity in tauopathy disorders, and suggest feasibility for novel therapeutic approaches targeting TRCs.