In the current study, an engineering design-based STEM activity was designed and implemented for 5th graders. The current activity is expected to provide guidance and perspective to teachers (practitioners) in designing and implementing an activity based on design-based learning, STEM activity, and engineering design process (EDP). At the same time, during the implementation of the activity, teachers and students experienced a STEM activity based on the EDP. In the fall semester of the 2022-2023 academic year, this STEM activity based on the engineering design process was planned for Friction force and water resistance in the 5th-grade middle school science course. Then, the activity was implemented in a class of 21 students. The activity was implemented in three class hours. This activity, titled "Let's Design a Water Slide Boat," aimed at designing a water slide boat that would be least affected by water resistance and friction force to improve students' engineering and design skills. This activity was based on NGSS and the objectives and outcomes set in the 5th-grade science curriculum of the Turkish Ministry of National Education.