Entangled relativity is a new theory of relativity that does not allow gravity to be treated separately from matter fields. Unlike general relativity, the theory therefore satisfies the last of Einstein's three original demands for an acceptable theory of relativity, which he named Mach's Principle. Indeed, unlike in general relativity, inertia cannot be defined ex nihilo in entangled relativity, because the definition of the theory demands the existence of matter in the first place. Apart from that, the theory does not assume any new ingredient with respect to general relativity: for instance, spacetime is described by a four-dimensional manifold, the metric tensor encodes the mechanical properties of space as well as the inertia of bodies and gravitation, and its curvature is generated by matter fields. In fact, the theory even possesses one less free parameter w.r.t. general relativity, as Newton's constant no longer is a fundamental constant in entangled relativity, but results from the primordial evolution of the universe. The theory also has the nice property to possess general relativity as one of its limits (for fairly generic situations when the density is not way higher than the central density of neutron stars).