We introduce a nanofluidic platform that can be used to carry out femtosecond electron diffraction (FED) and transmission electron microscopy (TEM) measurements in liquid samples or in-liquid specimens, respectively. The nanofluidic cell (NFC) system presented herein has been designed to withstand high sample refreshing rates (over one kilohertz), a prerequisite to succeed with FED experiments in our lab. Short beam paths, below 1 μm, in combination with ultrathin membranes (less than 100 nm thick) are necessary conditions for in-liquid FED and TEM studies due to the strongly interacting nature of electrons. Depending on the application, the beam path in our NFC can be tuned between 50 nm and 10 μm with ultrathin stoichiometric silicon nitride (Si3N4) windows as thin as 20 nm. Stoichiometric Si3N4 has been selected to reduce membrane bulging owing to its higher tensile stress and transparency in the UV-vis-NIR region to allow for laser excitation in FED experiments. Key design parameters and improvements made over previous NFC systems are discussed, and some preliminary electron images obtained by 200 kV scanning TEM are presented.