The streets of Seoul vary widely in width. Streets less than 12 meters of width are called alleyways. Alleyways are important in that these streets constitute neighborhoods where people live and carry out their daily lives. The total length of alleyways occupy 76.3% of all the streets in Seoul, meaning that the alleyway is the most important type of street. However, alleyways in Seoul are not in good condition. Since it is inevitable that pedestrians and vehicles share limited space of alleyways, there are numerous concerns such as insufficient pedestrian zones, potential collisions between people and cars, and increasing illegal parking. In 2022, while the law regulating roadway usage has been revised in a way to increase pedestrian safety, there is no consensus as to how alleyways should be designed and managed. Of course, there have been various design guidelines and manuals that addressed the issues of alleyways. However, those guidelines lack specific solutions that could be applied to improve alleyway designs in Seoul. Here we present the Seoul Alleyway Design Guideline. This is a design guideline that provides solutions to improve the conditions of alleyways of Seoul. We focus on three important elements of alleyway design: 1) parking space, 2) pedestrian safety, and 3) traffic operation and control. In particular, this guideline concerns how to allocate the limited space of alleyway among parking, pedestrians, and vehicles. Considering these three elements, various design alternatives have been developed, and those ideas can be applied differently according to the width of alleyways. This guideline shows the process of making decisions in allocating limited space for the three elements, and thereby determining an improvement plan for alleyways. In addition, we present graphic design examples of improved alleyway designs. Those examples show how real alleyways can be improved through design process presented in this guideline. We believe that it will help project managers who work on improving alleyways design. With this guideline at hands, public officials at local Gu district governments in Seoul will be benefited for years to come.