Optical activity in 3D-chiral media in the form of circular dichroism and birefringence is a fundamental phenomenon that serves as evidence of life forms and is widely used in spectroscopy. Even in 3D-chiral media exhibiting strong transmission optical activity, the reflective effect is weak and sometimes undetectable. Here we report that specular optical activity at structured interfaces can be very strong. Resonant polarization rotation reaching 25 • and reflectivity contrast exceeding 50% for oppositely circularly polarized waves are observed for microwaves reflected by a metasurface with structural elements lacking two-fold rotational symmetry. The effect arises at oblique incidence from a 3D-chiral arrangement of the wave's direction and the metasurface's structure that itself does not possess chiral elements. Specular optical activity of such magnitude is unprecedented. It is fundamentally different from the polarization effects occurring upon scattering, reflection and transmission from surfaces with 2D-chiral patterns. The scale of the effect allows applications in polarization sensitive devices and surface spectroscopies.Optical activity, that is the ability to rotate the polarization state of light (circular birefringence) and differential throughput of circularly polarized waves (circular dichroism), is a fundamental electromagnetic effect of importance in various fields, from analytical chemistry and crystallography to sensors and display applications. Optical activity is usually observed for transmission through structurally 3D-chiral (mirror-asymmetric) media, e.g. crystals of quartz or cinnabar, or solutions of 3D-chiral molecules like sugar or proteins. It is much less well-known that polarization rotation also occurs for reflection from chiral materials [1-3] and diffusive scattering from chiral liquids [4]. Just as conventional polarization rotation (transmission circular birefringence) is usually accompanied by different levels of transmission for right-handed and left-handed circularly polarized waves (transmission circular dichroism), polarization rotation in reflection (specular circular birefringence) also goes along with a difference in reflectivity levels for waves of opposite handedness (specular circular dichroism), which was observed, for example, for solutions of camphorquinone [5] and proteins [6] as well as thin films of chiral polyfluorene [7].Both specular circular birefringence and specular circular dichroism appear to be subtle phenomena, which are hardly acknowledged and remain of little practical importance even if enhanced through multiple reflections [8], see Fig. 1(a). Indeed, in the case of light transmission, optical activity accumulates over the propagation distance and can gain up to several hundred degrees per millimeter. In reflection, however, the phenomenon results from scattering of light by only few atomic/molecular layers located at the interface.In this letter we demonstrate giant specular optical activity for a single reflection. Specular circular birefringenc...