The seismic behavior of unreinforced masonry (URM) structures has usually been investigated by conducting laboratory tests. Such efforts, although useful in order to understand the response of single walls or subassemblages, cannot fully mimic the in situ seismic response of masonry buildings. In this study, in situ lateral load testing of a two-story full-scale URM building was conducted. First, forced vibration tests were performed to determine the vibration frequencies of the building. Afterwards, the building was tested by reacting against a braced steel frame built behind the test building and imposing one-way cyclic displacements. The building was pushed until 20% loss of lateral load capacity, which corresponds to a drift ratio of approximately 0.8%. The diagonal tension cracking of the walls was followed by sliding of the second-story concrete slab over the walls. A three-dimensional nonlinear finite-element analysis of the test building was conducted to simulate the behavior of the test building. The results showed that upon careful selection of the contact cohesion at the slab-wall interfaces, the numerical model provided reasonable estimations of the global structural behavior in terms of load-deformation response.