Topological spin structures such as magnetic skyrmions are of fundamental interest and promising for various types of applications in spintronics. Skyrmions have been predicted to emerge also in antiferromagnetic materials where they exhibit superior transport properties. They were experimentally revealed in synthetic antiferromagnets, however, still remain elusive in intrinsic atomic antiferromagnets. Here, we demonstrate how topological spin structures can be stabilized in an antiferromagnetic monolayer. Using spin-polarized scanning tunneling microscopy, we observe an antiferromagnetic spin spiral in the Mn monolayer and a collinear antiferromagnetic state in the Mn double-layer on Ta(110). Near the boundary to the double-layer spin structures equivalent to half of an antiferromagnetic skyrmion form in the monolayer as revealed in combination with first-principles calculations and micromagnetic simulations. Our simulations show how the topological state in antiferromagnetic material systems can be controlled by the configuration within a lateral heterostructure, resulting in trivial non-coplanar states or antiferromagnetic skyrmions.