Aim and background: The quest for optimal restorative material balancing function, biocompatibility, and durability is ongoing. Among these materials, glass ionomer cement (GIC) remains unique due to its fluoride release and chemical adhesion. This study explores incorporating micro- and nano-hydroxyapatite (HA) into GC Hybrid to improve tensile strength, shear bond strength, and microleakage. Materials and methods: Thirty-three extracted human primary molars were divided into three groups: GC Hybrid (control), GC Hybrid + micro-HA, and GC Hybrid + nano-HA. Tensile strength, shear bond strength, and microleakage were evaluated using standard methods. Results: The nano-HA group showed significantly higher tensile (8.46 ± 1.42, p = 0.00) and shear bond strength (9.19 ± 1.82, p = 0.01), with no microleakage. Moderate improvements were seen with micro-HA, while the control had the lowest performance. Conclusion: GC Hybrid with nano-HA significantly enhances bonding, mechanical properties, and reduces microleakage. Clinical significance: Nano-HA lowers the risk of secondary caries, making it superior to standard GC Hybrid. © 2025 Elsevier B.V., All rights reserved.