Technical solutions for sealing and covering seismic joints may induce dynamic interactions between as-built adjacent structures. Moreover, noise modes due to dynamic interactions make modal parameter identification complex. In this paper, ambient vibration tests were conducted to obtain the actual dynamic characteristics of a reinforced concrete office building, whose seismic joints were formed by cantilever members. The tested building was divided into the main building, eastern building and western building. Tests were conducted in each structural part separately. Four modes of the main building and eastern building, and two modes of the western building were identified using the frequency domain decomposition method. The results show that due to the dynamic interactions between adjacent structures caused by the infill of the seismic joints, natural frequencies of a structure were mixed together with those due to the interactions with the adjacent structures. Modes of each building from the finite element models agreed well with the identification results, except that the third natural frequencies of the main building and eastern building were smaller than the identified ones, because the integrity of the whole building was enhanced by the dynamic interactions. Therefore, it should not always be assumed that adjacent structures with seismic joint are independent.