Abstract: To address the service safety issue of welded structures in marine and polar engineering under extreme low-temperature environments, low-cycle fatigue tests on undermatched (UM) welded joints of 10CrNi3MoV high-strength steel were conducted at room temperature (RT) and low temperatures (−20 ℃, −40 ℃, and −60 ℃). Microstructural observation, monotonic tensile tests, Charpy impact tests, strain-controlled low-cycle fatigue tests, and fatigue crack growth tests were combined to systematically analyze the effects of temperature on the cyclic stress response, hysteresis behavior, strain–life relationship, and energy–life relationship of the welded joints, and the fatigue ductile-to-brittle transition behavior of the undermatched joints was discussed in combination with fracture morphology. The results show that with the decrease of temperature, the undermatched joints exhibit cyclic softening and non-Masing characteristics; their fatigue crack initiation resistance is relatively low; the plastic strain energy density at half-life can effectively characterize the fatigue life of the joints at low temperatures; the fatigue ductile-to-brittle transition temperature of the undermatched joints is approximately −40 ℃, which is higher than the impact ductile-to-brittle transition temperature. The study provides an experimental basis for the fatigue-resistant design of polar ships and marine engineering structures.