Abstract Interface superconductivity, realized in multiple artificial crystalline heterostructures, is one of the most exciting directions to search for high-temperature superconductivity. In this work, we prepare bulk (FeSe) _η (SrTiO₃)_1−η multi-grain composites by a simple facile liquid-phase compaction method using a spark-plasma-sintering technique. Combining transmission electron microscopy/scanning electron microscopy and x-ray diffraction investigations, we demonstrate that the composites consist of micron-scale SrTiO₃ grains surrounded by [001]-compressed β-FeSe grains. Transport measurements for the composites with FeSe mole fraction η > 0.06 reveal that two superconducting channels, one T _c ∼ 13 K phase from FeSe/SrTiO₃ interfaces and another T _c ∼ 7 K phase from FeSe grains, cooperatively induce macroscopic superconducting behavior with isotropic upper critical fields above 40 T. This work points out a straightforward method to enhance T _c in the multi-grain (FeSe) _η (SrTiO₃)_1−η composites by reducing the crystalline grains to nanoscale and finely tuning the stoichiometries of FeSe and SrTiO₃.