The Royal Microscopical Society (RMS) is a learned society for the promotion of microscopy. It was founded in 1839 as the Microscopical Society of London making it the oldest organisation of its kind in the world. In 1866, the society gained its royal charter and took its current name. Founded as a society of amateurs, its membership consists of individuals of all skill levels in numerous related fields from throughout the world. Every year since 1852, the society has published its own scientific journal, the Journal of Microscopy, which contains peer-reviewed papers and book reviews. The society is a registered charity that is dedicated to advancing science, developing careers and supporting wider understanding of science and microscopy through its Outreach activities.

Probably the society's greatest contribution is its standardised 3x1 inches microscope glass slides in 1840, which are still the most widely used size today and known as the "RMS standard".

The Royal Microscopical Society is a member of the Foundation for Science and Technology, the Biosciences Federation, the European Microscopy Society and the International Federation of Societies for Microscopy.