MolecularWebXR is a new web-based platform for education, science communication and scientific peer discussion in chemistry and biology, based on modern web-based Virtual Reality (VR) and Augmented Reality (AR). With no installs as it is all web-served, MolecularWebXR enables multiple users to simultaneously explore, communicate and discuss concepts about chemistry and biology in immersive 3D environments, by manipulating and passing around objects with their bare hands and pointing at different elements with natural hand gestures. Users may either be present in the same physical space or distributed around the world, in the latter case talking naturally with each other thanks to built-in audio. While MolecularWebXR offers the most immersive experience on high-end AR/VR headsets, its WebXR core also supports participation on consumer devices such as smartphones (with optional cardboard goggles for enhanced immersion), computers, and tablets. MolecularWebXR includes preset VR rooms covering topics in general, inorganic, and organic chemistry, as well as biophysics, structural biology, and general biology. Users can also add new content via the PDB2AR tool. We demonstrate MolecularWebXR's versatility and ease of use across a wide age range (12–80) in fully virtual and mixed real-virtual sessions at science outreach events, undergraduate and graduate courses, scientific collaborations, and conference presentations. MolecularWebXR is available for free use without registration at https://molecularwebxr.org. A blog post version of this preprint with embedded videos is available at https://go.epfl.ch/molecularwebxr-blog-post.