  A new beam dump experiment that utilizes the beam of future high energy
electron-positron colliders could be an excellent avenue to search for dark
sector particles due to its unprecedented high energy and intensity. We
consider heavy neutral leptons (HNLs) as a specific example to demonstrate the
sensitivity of searches for dark sector particles at future electron-positron
collider beam dump experiments. This includes the study of the reach at the
International Linear Collider (ILC), the Cool Copper Collider ($\rm C^3$), and
the Compact Linear Collider (CLIC). We comprehensively examine the HNL
production and detector acceptance at these electron beam dump experiments. We
show that these experiments will probe a large range of HNL parameter space,
not yet probed by past experiments. These experiments will be complementary to
other proposed experiments such as proton beam dump experiments, neutrino
experiments, and LHC auxiliary detectors. Our study also motivates a more
detailed analysis of heavy meson productions in high-energy electron-nucleon
collisions in thick targets.
