Abstract: This study presented a novel nutrient fortification strategy that involved incorporating a walnut oil-based high internal phase emulsion (HIPEs), which was rich in polyunsaturated fatty acids (PUFAs) and stabilized by yeast protein particles (YPs), into an infant rice cereal gel for 3D printing applications. Yeast protein was modified with microbial transglutaminase (MTGase), which resulted in a significant increase in hydrophobicity (the three-phase contact angle increased from 69.2° to 77.8°) and consequently enhanced its interfacial stabilization capacity. Utilizing the modified YPs as a stabilizer, HIPEs with a high internal phase volume fraction of up to 75% were successfully prepared. The results indicated that increasing the concentration of yeast protein particles (YPs) (from 1.0 to 6.0 wt%) significantly reduced the droplet size of HIPEs (from 43.18 μm to 6.57 μm), increased the absolute zeta potential value, and promoted the formation of a dense interfacial protein layer. These modifications collectively enhanced the physical and oxidative stability of the emulsion, effectively inhibiting the oxidation of PUFAs in the walnut oil. The resulting HIPEs were then incorporated as a functional ingredient into an infant rice paste gel to systematically investigate their effects on the printability of the cereal-based 3D printing ink, as well as the textural properties and swallowing safety, according to the International Dysphagia Diet Standardisation Initiative (IDDSI), of the printed constructs. The results demonstrated that the addition of HIPEs significantly refined the microstructural network of the rice paste gel, consequently enhancing the printing precision and shape fidelity. Texture Profile Analysis (TPA) revealed that the sample with an 8% addition of HIPEs exhibited significantly enhanced hardness, springiness, cohesiveness, and chewiness compared to the control group, thereby meeting the textural requirements for easy-to-swallow foods. This study confirmed that YPs-stabilized HIPEs served as an excellent delivery vehicle for PUFAs and demonstrated substantial potential for improving both the nutritional value and physical properties of 3D-printed infant foods. This research provided a novel scientific foundation for the development of personalized, nutritionally fortified, and swallow-safe complementary foods.