In this paper, we consider the asymptotic behavior of a semilinear elliptic problem in a thin two‐composite domain with an imperfect interface, where the flux is discontinuous. For this thin domain, both the height and the period are of order
. We first use Minty–Browder theorem to prove the well‐posedness of the problem and then apply the periodic unfolding method to obtain the limit problems and some corrector results for three cases of a real parameter
,
and
, respectively. To deal with the semilinear terms, the extension operator and the averaged function are used.