Concrete-filled tubular (CFT) frames have been widely applied to high-rise buildings in Japan. However, there is a need for more cost-effective beam—column connection details, especially for square tube columns. To meet this need, a beam—column connection stiffened with two interior T-shaped diaphragms was investigated experimentally through both monotonic loading connection tests and cyclic loading frame tests. These test results could be estimated properly by using proposed design formulas that consider the yield mechanism of the connection.