This work evaluated the structure and quality parameters of dough and cookies prepared using fermented cassava flour, soy protein concentrate, and glucomannan. Glucomannan was incorporated as a fat replacement. The levels of fat replacement were 0%, 10%, 20%, and 30% (CS100, GL90, GM80, and GH70, respectively) relative to the original fat content. Rheological analysis showed that the loss tangent of GH70 dough was the lowest (0.39) with an angular frequency of 0.62 rad/s. Thus, glucomannan affected the dough and caused it to exhibit more solid-like behaviors. Glucomannan covered the starch granules, which mimicked that of fat. A complex interaction was confirmed among starch, glucomannan, and protein in the dough. Glucomannan decreased dough hardness and adhesiveness but significantly increased (p < 0.05) cookie hardness, crispiness, and spread ratio. Dough with low rheological properties, adhesiveness, and hardness will produce good-quality cookies. Glucomannan can be applied as a fat replacer in cookies.