<p>Microwave isolators, developed after World War II, are essential non-reciprocal devices widely used to minimize signal reflections and interference across various applications, including mobile base stations, satellite communications, radar systems, magnetic resonance imaging and industrial microwave heating. A typical commercial microwave isolator provides 20 dB of isolation, reducing the backward power by two orders of magnitude. Although higher isolation is always desired for systems that require greater power or lower noise, such as superconducting quantum computing, further reduction in the backward signal will inevitably lead to an unacceptable degradation in the forward transmission in traditional designs. Here we introduce the principle of a topological isolator, based on a unique one-way edge waveguide that spatially separates forward and backward waves, allowing for the complete absorption of the backward-propagating mode without compromising any forward signal. This ideal isolation mechanism produces an unprecedented isolation level, analytically derived to be 200 dB within a single-wavelength-size device. It is limited only by the evanescent fields within the topological bandgap in the ferrite material that spans two octaves around 10 GHz. We experimentally demonstrate this topological isolator in a stripline configuration with a minimal insertion loss of 1 dB and a backward signal deeply attenuated to the instrument noise floor. This results in an ultrahigh isolation exceeding 100 dB—an eight-orders-of-magnitude improvement over conventional counterparts. Our work not only paves the way for higher-performance isolators in the aforementioned technologies but also sets the stage for innovation in a variety of related microwave components.</p>