  Gravitational wave (GW) signals arising from binary neutron star mergers
offer new, sensitive probes to ultralight mediators. Here we analyze the GW
signals in the GW170817 event detected by the LIGO/Virgo collaboration to
impose constraints on the ultralight isospin-violating mediator that has
different couplings to protons and neutrons. Neutron stars, which primarily
consist of neutrons, are the ideal places to probe the isospin-violating
mediator. Such a mediator can significantly alter the dynamics of the binary
neutron star mergers, through both the long-range Yukawa force and the new
dipole radiation. We compute the gravitational waveform by taking into account
the new physics effects due to the isospin-violating mediator and use the
Bayesian inference to analyze the gravitational wave data in the GW170817
event. We find that although the current fifth force experiments (including
MICROSCOPE and EW) often provide more stringent constraints than the GW170817
data, in the parameter space where the isospin-violating force is completely
screened by the Earth (namely, the Earth is charge neutral under this force),
the GW170817 data offer the leading constraints: the upper bound on the neutron
coupling is $f_n \lesssim 10^{-19}$ in the mediator mass range of
$\simeq(3\times10^{-16},\,5\times10^{-14})$ eV.
