Fracture Mechanics is an extensively developed discipline for a wide range of materials used in scientific and engineering applications. However, the thorough understanding and characterization of fracture phenomena in many soil and rock-type materials are still under development. This work aims to characterize the suction-dependent fracture response in undisturbed clay soils, which will lead to identifying a ductile–brittle behavior threshold. The undisturbed condition will provide information on the behavior of the soil in its natural state. For this purpose, a new experimental procedure is presented in order to obtain a closed-form relationship between mode I fracture toughness and a suction range for the Guadalquivir blue marl. In order to achieve this objective, an experimental campaign was carried out using a series of semi-circular specimens subjected to the three-point bending test with different suctions. The corresponding loading–displacement curves from the tests are qualitatively analyzed to identify the fracture response of this material, and the linear regression technique is used to finally obtain the desired relationship. Current results show that an increasing value of suction increases both the maximum load and the stiffness, yielding a more brittle behavior associated with an increase. Finally, an alternative expression for the determination with respect to that proposed by Kuruppu and coauthors (Kuruppu et al., 2013) is presented, which involves the explicit inclusion of the suction variable for the more brittle behavior, limited by the threshold found. This provides a new perspective on how moisture content affects the fracture mechanics of clays.