This communication presents a compact leaky-wave antenna providing circular polarization from a dual-mode hollow waveguide (WG). The monolithic structure is based on a rectangular metallic guide that is perforated with crossed slots. The antenna is fed by a standard WR42 port, which is connected to the leaky WG by a transition that allows for dual-mode excitation. The axial ratio (AR) is determined by the size of the WG section and the dimensions of the crosses. In order to preserve the polarization purity, the geometry of the first radiating elements is modulated. Designed at K-band, the present proposal is suitable for space applications since it relies on a full-metal structure. A design methodology is proposed based on the dual-mode dispersion analysis. Selective laser melting is used to monolithically 3-D print the antenna. Measurements are in good agreement with simulations and the AR is 0.92 dB at 20.5 GHz.