We propose the use of trapped ions for detection of millicharged dark matter. Millicharged particles will scatter off the ions, giving a signal either in individual events or in the overall heating rate of the ions. Ion traps have several properties which make them ideal detectors for such a signal. First, ion traps have demonstrated significant isolation of the ions from the environment, greatly reducing the background heating and event rates. Second, ion traps can have low thresholds for detection of energy deposition, down to $sim text{neV}$. Third, since the ions are charged, they naturally have large cross sections for scattering with the millicharged particles, further enhanced by the low velocities of the thermalized millicharges. Despite ion-trap setups being optimized for other goals, we find that existing measurements put new constraints on millicharged dark matter which are many orders of magnitude beyond previous bounds. For example, for a millicharge dark matter mass $m_Q=10~textrm{GeV}$ and charge $10^{-3}$ of the electron charge, ion traps limit the local density to be $n_Q lesssim 1 , textrm{cm}^{-3}$, a factor $sim 10^8$ better than current constraints. Future dedicated ion trap experiments could reach even further into unexplored parameter space.
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