Na5InSi4O12 has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It is isotypic to Na5ScSi4O12. The structure consists of 12-membered single rings of corner-sharing SiO4 tetrahedra linked together via corner sharing by single InO6 octahedra to form a 3-D framework that delimits two types of channels along the c-axis. There are six independent sodium sites. The Na atoms within the 12-ring channels are immobile and the Na atoms within 7-ring channels are highly mobile. The 29Si MAS NMR exhibits one Q3 and one Q4 resonance, each of which is split into two peaks because of inequivalent chemical environments caused by partially occupied Na sites in the 7-ring channels. Na ion conductivity measurements on a pressed pellet of the title compound were performed. The Arrhenius plot of dc conductivity shows a change in slope near 220 °C. We attribute this to the activation of the Na site that is situated between 7-ring channels and may serve as a connecting link between the channels. Crystal data: trigonal, space group R-3c (No. 167), a = 21.7158(9) Å, c = 12.4479(7) Å, and Z = 18.