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This paper presents a test campaign concerning the fatigue behavior of the bird-beak square hollow section (SHS) gap K-joints subjected to brace in-plane force. A total of four bird-beak SHS gap K-joint specimens were tested. Prior to the high-cycle fatigue tests, static in-plane force was applied to the braces of K-joint to determine the hot-line strain distributions and the stress concentration factors (SCFs) via extrapolation. Based on the results of static loading tests, the amplitude of cyclic load can be further determined. Subsequently, high-cycle fatigue tests were performed on the square and diamond bird-beak SHS gap K-joints to investigate the fatigue behavior from different perspectives, such as failure mode, fatigue life, crack propagation, and stiffness degradation. The test results demonstrate that the brace crown region always possessed the most severe stress concentration, which coincided with the location of crack initiation. This implies that the experimentally obtained SCFs can accurately predict the crack initiation location. The test fatigue lives of bird-beak gap K-joints were compared with the predictions by the International Institute of Welding (IIW) Srhs–N design curves originally developed for the conventional joints. [ABSTRACT FROM AUTHOR]