Time‐series image capture of in vitro 3D spheroidal cancer models embedded within an extracellular matrix affords examination of spheroid growth and cancer cell invasion. However, a customizable, comprehensive and open source solution for the quantitative analysis of such spheroid images is lacking. Here, the authors describe INSIDIA (INvasion SpheroID ImageJ Analysis), an open‐source macro implemented as a customizable software algorithm running on the FIJI platform, that enables high‐throughput high‐content quantitative analysis of spheroid images (both bright‐field gray and fluorescent images) with the output of a range of parameters defining the spheroid “tumor” core and its invasive characteristics.