Damage or functional failure of vital organs remains a major clinical challenge, while the availability of donor organs for transplantation is severely limited. As a result, tissue engineering has emerged as a promising strategy for organ replacement; however, conventional top-down tissue engineering, which employs scaffolds to provide three-dimensional growth environments, cannot ensure precise cell positioning, restricting its applicability to complex and heterogeneous tissues. In contrast, bottom-up strategies that assemble spheroids or organoids as modular building blocks offer a more effective route to organ-like constructs. Nevertheless, they suffer from low reproducibility because of spontaneous cell self-assembly. Three-dimensional bioprinting provides a promising solution for the reproducible fabrication of multicellular organ building blocks (OBBs). At the same time, while extrusion-based bioprinting offers high reproducibility, its limited dimensional accuracy has restricted its use for fabricating OBBs that require both precise microarchitectures and reliable assembly. Here, we address this limitation by introducing a strategy in which bioinks are directly bioprinted within three-dimensionally printed molds, enabling the formation of OBBs with well-defined geometries and controlled spatial organization. By combining mold-guided bioprinting with multimaterial preset extrusion, we demonstrated the fabrication of heterogeneous OBBs with microscale architectures while preserving the modularity essential for bottom-up assembly. This approach resolves the conventional trade-off between structural precision and assembly-based scalability, allowing the construction of large tissue constructs with hierarchical vascular networks. Overall, this work presents a 3D bioprinting-based OBB fabrication strategy that integrates precision manufacturing with bottom-up tissue assembly, offering a reproducible and scalable framework for bioartificial organ engineering.