Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are age-related neurodegenerative disorders with shared genetic etiologies and overlapping clinical and pathological features. Here we studied a novel ALS/FTD family and identified the P362L mutation in the low-complexity domain (LCD) of T cell-restricted intracellular antigen-1 (TIA1). Subsequent genetic association analyses showed an increased burden of TIA1 LCD mutations in ALS patients compared to controls (p = 8.7 × 10−6). Postmortem neuropathology of five TIA1 mutations carriers showed a consistent pathological signature with numerous round, hyaline, TAR DNA-binding protein 43 (TDP-43)-positive inclusions. TIA1 mutations significantly increased the propensity of TIA1 protein to undergo phase transition. In live cells, TIA1 mutations delayed stress granule (SG) disassembly and promoted the accumulation of non-dynamic SGs that harbored TDP-43. Moreover, TDP-43 in SGs became less mobile and insoluble. The identification of TIA1 mutations in ALS/FTD reinforces the importance of RNA metabolism and SG dynamics in ALS/FTD pathogenesis. •Mutations affecting the low-complexity domain of TIA1 cause ALS and ALS-FTD•ALS-linked TIA1 mutations share a neuropathological TDP-43 signature•TIA1 mutations promote phase separation and impair stress granule dynamics•TDP-43 recruited to poorly dynamic stress granules becomes immobile and insoluble Mackenzie et al. report the identification of mutations in TIA1 as a novel cause of ALS and ALS-FTD. ALS-linked mutations perturb phase transitions of TIA1, which impair the dynamics of stress granules and indirectly promote TDP-43 insolubility.