The integration of 3D printing technologies into the production of shape memory alloys (SMAs) has opened new windows in engineering and enabled the manufacture of complex, customized components with unprecedented geometrical complexity and precision. This chapter explores the key role of alloy design in the 3D printing of SMAs, shedding light on the intricate process of tailoring material compositions to achieve desired thermomechanical properties. Beginning with an introduction to the state of the art in additive manufacturing (AM) of SMAs, the chapter delves into the critical considerations for selecting and optimizing alloy compositions for 3D printing. It explores the principles of alloy design, discussing how precise control over composition and microstructure impacts the shape memory effect, superelasticity, and other key properties. The role of computational approaches and cutting-edge machine learning techniques for alloy design is also examined, alongside in situ alloy modification during the AM process. Through case studies and examples, the chapter demonstrates the real-world applications of designed SMA alloys in diverse fields, including aerospace, automotive, biomedical, and robotics. It concludes by addressing current challenges in alloy design and offering insights into the future of customized SMAs through 3D printing, highlighting the potential for breakthroughs in materials science and engineering. In essence, “alloy design for 3D-printed SMA” provides an in-depth exploration of the pivotal role that alloy design plays in realizing the transformative potential of 3D-printed SMAs. This chapter serves as a valuable resource for researchers, engineers, and enthusiasts seeking to push the boundaries of material science and AM, ultimately paving the way for innovative solutions in an ever-evolving technological landscape.