We discuss the effect of the nanometric grain size on the behavior of the electrical and magnetoresistive response of La_0.8K_0.2−x□_xMnO_3−δ (x = 0 and 0.1) nanocrystalline samples that were prepared by a sol–gel method. The results from transport and magneto-transport measurements evidence a robust dependence on the nanometric grain size. The temperature dependence of the resistivity was evaluated using different transport models. The results reveal a field-dependent minimum of the resistivity in the low-temperature region, which can be described in terms of intergranular spin-polarized tunneling. Remarkably, a considerable increase of the magnetoresistance (MR) with the decrease of nanoparticle size was found, which might open a new way for the search for potential candidates for magnetoresistive devices. Besides, the magnetic field dependence of the MR was also analyzed, and a distinct drop of MR at low fields was noticed. This behavior was primarily explained by the spin-polarized tunneling transport of conduction electrons across grain boundaries.