Plasmonic electrochromism, the dependence of the colour of plasmonic materials on the applied electrical potential, has been under the spotlight recently as a key element for the development of optoelectronic devices and spectroscopic tools. In this review, we focus on the electrochromic behaviour and underlying mechanistic principles of plasmonic metal nanoparticles, whose localised surface plasmon resonance occurs in the visible part of the electromagnetic spectrum, and present a comprehensive review on the recent progress in understanding and controlling plasmonic electrochromism. The mechanisms underlying the electrochromism of plasmonic metal nanoparticles could be divided into four categories, based on the origin of the LSPR shift: (1) capacitive charging model accompanying variation in the Fermi level, (2) faradaic reactions, (3) non-faradaic reactions, and (4) electrochemically active functional molecule-mediated mechanism. We also review recent attempts to synchronise the simulation with the experimental results and the strategies to overcome the intrinsically diminutive LSPR change of the plasmonic metal nanoparticles. A better understanding and controllability of plasmonic electrochromism provides new insights into and means of the connection between photoelectrochemistry and plasmonics as well as future directions for producing advanced optoelectronic materials and devices.