Ruthenium nitride thin films with ZnS type cubic structure are successfully and reproducibly deposited by pulsed-DC magnetron sputtering. Their electrochemical behavior as negative electrode for Li-ion microbatteries is investigated by cyclic voltammetry and galvanostatic experiments. The influence of the film thickness and the rate capability are reported and discussed. A specific capacity of about 400 mA h g−1 is proved to be stable at C/2 over 20 cycles at a mean voltage of 0.25 V vs. Li+/Li and attractive specific capacities varying from 100 to 300 μA h cm−2 can be reached depending on the thickness in the range 250 nm–850 nm. XRD, SEM, HRTEM, electron diffraction and EELS are used to understand the electrochemical mechanism of RuN with lithium. Electrochemical properties and structural characterization support the occurrence of a reversible conversion reaction of RuN into Ru metal and Li3N.