Yeast ataxin-2, also known as Pbp1, senses the activity state of mitochondria in order to regulate TORC1. A domain of Pbp1 required to adapt cells to mitochondrial activity is of low sequence complexity. The low-complexity (LC) domain of Pbp1 forms labile, cross-β polymers that facilitate phase transition of the protein into liquid-like or gel-like states. Phase transition for other LC domains is reliant upon widely distributed aromatic amino acids. In place of tyrosine or phenylalanine residues prototypically used for phase separation, Pbp1 contains 24 similarly disposed methionine residues. Here, we show that the Pbp1 methionine residues are sensitive to hydrogen peroxide (H2O2)-mediated oxidation in vitro and in living cells. Methionine oxidation melts Pbp1 liquid-like droplets in a manner reversed by methionine sulfoxide reductase enzymes. These observations explain how reversible formation of labile polymers by the Pbp1 LC domain enables the protein to function as a sensor of cellular redox state. Display omitted •Phase separation of the yeast ataxin-2 methionine-rich LC domain is sensitive to H2O2•The dissolved droplets are specifically revived by methionine sulfoxide reductases•Methionine to tyrosine variants confer resistance to H2O2in vitro and in cells•Yeast ataxin-2 senses cellular redox state to modulate TORC1 activity Many proteins that are able to undergo phase transitions contain repeats rich in hydrophobic residues. By contrast, yeast ataxin-2 condensates rely on an abundance of methionines, providing a means for redox-sensitive regulation of its material properties.