This study aimed to analyze and classify the lower body shapes of underweight women in their twenties, defined as having a body mass index (BMI) below 18.5 kg/m², using three-dimensional body scan data from the 8th Size Korea Anthropometric Survey provided by the Korean Agency for Technology and Standards. The goal was to offer foundational data for developing well-fitted lower garments tailored to the specific physical characteristics of this demographic. As a result of cluster analysis, three distinct lower body types were identified. Type 1, referred to as the “long and small stork-leg type,” is characterized by a long and high lower body, with extended hip and crotch lengths. Individuals in this group have slender calves and ankles, a very slim waist and abdomen, and relatively small hips and thighs. Type 2, the “long and voluminous pillar-leg type,” also features a long and high lower body, but with thick calves and ankles, a larger waist and abdomen, and significantly voluminous hips and thighs. However, the crotch and hip lengths are remarkably short. Type 3, the most common among the participants, is the “short and average ladder-leg type,” characterized by a very short lower body and thick calves and ankles. These findings underscore the distinct lower body characteristics of underweight women compared to standard body types, indicating the need to develop new pattern drafting systems specifically for this group. Underweight women tend to have relatively longer vertical body dimensions; thus, standard ratios for pants length, knee position, crotch depth, and hip level may not be appropriate. New proportion systems are required for improved garment fit. This study is meaningful in that it classified the lower body types of a significant portion of underweight women in their twenties and highlighted the necessity of customized clothing development. Furthermore, follow-up studies should develop actual pants pattern-making methods based on these findings to enhance fit and comfort.