Unidirectional radiation is important for various optoelectronic applications, such as lasers, grating couplers and optical antennas. However, almost all existing unidirectional emitters rely on the use of materials or structures that forbid outgoing waves-that is, mirrors, which are often bulky, lossy and difficult to fabricate. Here we theoretically propose and experimentally demonstrate a class of resonances in photonic crystal slabs that radiate only towards one side of the slab, with no mirror placed on the other side. These resonances, which we name 'unidirectional guided resonances', are found to be topological in nature: they emerge when a pair of half-integer topological charges(1-3) in the polarization field bounce into each other in momentum space. We experimentally demonstrate unidirectional guided resonances in the telecommunication regime by achieving single-side radiative quality factors as high as 1.6 x 10(5). We further demonstrate their topological nature through far-field polarimetry measurements. Our work represents a characteristic example of applying topological principles(4,5) to control optical fields and could lead to energy-efficient grating couplers and antennas for light detection and ranging. Unidirectional radiation is achieved in a photonic crystal slab without the use of mirrors by merging a pair of topological defects carrying half-integer charges.