Cotton, a major crop worldwide, is harvested in mechanized production systems once at the end of the growing season. To facilitate harvest and maximize fiber quality, the plants are typically defoliated when about 60% of the cotton bolls are open. Due to non-uniform maturation, the bolls that have opened early expose their fiber to weather until harvest, commonly for weeks, degrading fiber quality. Furthermore, high capacity harvesting machines are heavy, potentially compacting the soil that in turn reduces hydraulic conductivity in the wheel tracks and reducing yield. Robotic harvesting with smaller machines brings about the possibility of multiple harvests during the growing season while enabling them to pick the seed cotton soon after the boll opens, preserving fiber quality. Smaller machines would also be less likely to substantially compact the soil. Therefore, research has been conducted to enumerate and address multiple challenges associated with the design of a robotic cotton harvester. The particular focus of the research reported herein was on the design of a robotic end-effector for picking seed cotton from the open boll of a non-defoliated cotton plant. Various design concepts were considered, and some were built as prototypes and experimentally assessed. The design was selected as optimal was: a three-finger, moving pinned belt, underactuated end-effector. A refined prototype of the end-effector was indoor tested on a robotic platform with a computer-controlled three-degree-of-freedom manipulator. The end-effector could pick 56-85% of the seed cotton from a boll with a picking time of 4 s for a simple and less efficient system to 18 s for a controlled-movement and more efficient system. Further implications of this study will include adding a depth sensor on the robot to detect and localize cotton bolls and manipulate arm autonomously.