Antenna pattern measurement generally requires an anechoic chamber with a single line-of-sight (LOS) propagation. However, it may not be satisfied due to cost and size limitations, leading to a nonanechoic environment (with echoes) and a distorted measured antenna pattern. To address this challenge, this article proposed an echo suppression method using phaseless data at a single frequency. With the proposed method, one can discriminate the multipath characteristic convoluted with the actual antenna pattern by measuring the antenna gain pattern at multiple spatial locations. Different from the state-of-the-art methods requiring phase information, the proposed method is an amplitude-only technique, thus enabling cost-effective and true over-the-air (OTA) measurement for the wireless device. Furthermore, we derived a systematic guideline for the proposed method to improve measurement efficiency based on numerical simulations. The proposed method is experimentally validated by placing a metal plate in the chamber to emulate a nonanechoic environment, and a good agreement between the true antenna pattern and the reconstructed antenna gain pattern is found. Both numerical and experimental validations demonstrated the feasibility and robustness of the proposed technique.