Concepts and technologies for dynamic cyclic extension (“CE”) for Fast Access to Subscriber Terminals (“G.Fast”) are described. According to one aspect described herein, a system can synchronize a G.Fast modem with the default CE value, measure an upstream signal attenuation of a G.Fast cable in a G.Fast circuit to obtain an upstream signal attenuation value, determine a new CE value based upon the upstream signal attenuation value, and determine if the new CE value is not equal to a default CE value. In response to determining that the new CE value is not equal to the default CE value, the system can update and apply a CE value for the G.Fast cable in the G.Fast circuit to the new CE value. If, however, the new CE value is equal to the default CE value, the system can instead apply the default CE value.