<p>A real-time transmission electron microscopy (TEM) study of electron beam–induced reduction of mono- and bimetallic metal ions encapsulated within dendrimers, leading to the formation of dendrimer-encapsulated nanoparticles (DENs), is demonstrated. Traditionally, DENs are prepared by coordinating metal ions to the tertiary amines of dendrimers, followed by chemical reduction. In this work, fourth- and sixth-generation hydroxyl- and amine-terminated PAMAM dendrimers loaded with Pt²⁺ and/or Pd²⁺ ions were directly reduced under electron beam irradiation, enabling in situ observation of nanoparticle nucleation and growth with particle size ranging from 1 to 2&#xa0;nm. This facile approach produces uniformly sized nanoparticles directly on TEM grids, offering promising opportunities for applications in catalysis and sensing through well-dispersed active surfaces.</p>