  This paper describes the liquid scintillator veto system for the COSINE-100
dark matter experiment and its performance. The COSINE-100 detector consists of
eight NaI(Tl) crystals immersed in 2200~L of linear alkylbenzene-based liquid
scintillator. The liquid scintillator tags between 65 and 75\% of the internal
$^{40}$K background in the 2--6 keV energy region. We also describe the
background model for the liquid scintillator, which is primarily used to assess
its energy calibration and threshold.
