I will discuss the structure of the tetraquark candidate Zc(3900)
from the meson-meson coupled-channel scattering on the lattice.
The Zc(3900) is experimentally reported as a peak in the pi J/psi invariant mass
in the Y(4260)-->pi pi J/psi decay.
To understand the nature of the Zc(3900), it is most important
to extract the coupled pi J/psi - rho eta_c - anti-D D interactions faithful to QCD S-matrix.
For this purpose, we employ the coupled-channel HAL QCD method,
and the s-wave coupled-channel meson-meson potential relevant to the Zc(3900) is extracted.
Scattering observables such as invariant mass spectra
and the pole position of the S-matrix on the complex energy plane are calculated.
Our results show that the Zc(3900) is not a conventional resonance
but a threshold cusp associated with the opening of the anti-D D threshold.