Expansive clays cause problems throughout the earth, in particular for the locations in arid or semi-arid climate. Ankara (Turkey) is in semi-arid climate and change in moisture content of the reddish-brown expansive Ankara clay causes swell-shrink related movements and damages on lightweight structures. Over-estimation of undrained shear strength of an expansive clay without considering swell amount can lead to failures (i.e., bearing capacity, slope stability, and pile capacity). Since the existence of water significantly affects the engineering properties of the swelling soils, the effect of swell on the undrained shear strength of Ankara clay was researched in this laboratory study. To verify this effect, tests were done on three expansive Ankara clay samples with distinct degree of expansiveness. Expansive clay samples were taken from several sites in Ankara. Before swell-shear test cycle, index properties of the samples were determined. Swelling of the specimens in designed molds and shearing process including 0%, 10%, 15%, 20%, 25%, 50%, and 75% of ultimate vertical swell and 100% swell were followed it. It has been determined that undrained shear strength of the expansive Ankara clay is considerably affected by the amount of swell depending on the test results. The undrained shear strength decreases with increasing water content. It was observed that the shear strength of the specimens was decreased to nearly 70% of its initial value, even at the 10% of ultimate swell, and the undrained shear strength of expansive Ankara clay specimens was lost to a degree of almost 90% of their initial value (i.e., almost complete loss of strength due to swelling), this is one of the major factors causing failure of structures in/on the expansive Ankara clay due to wetting in the depth of active zone.