Overlaying two two-dimensional (2D) lattices with different periodicities or a relative twist angle gives rise to a larger (quasi-) periodic lattice called a moiré pattern. In van der Waals (vdW) heterostructures, the moiré pattern has recently been shown to display interesting novel physical phenomena. For instance, twisted bilayer graphene near the “magic angle” of 1.1° hosts flat electronic bands and strongly correlated states such as unconventional superconductivity [1, 2]. On the other hand, the moiré pattern has been found to both localize and tune interlayer excitons in transition metal dichalcogenides (TMDCs) heterostructures with small twist angle [3-5]. Such moiré heterostructures are generally produced by mechanical exfoliation and stacking to precisely control the twist angle and moiré periodicity. However, this laborious method leads to interfacial contamination and twist angle inhomogeneities and is inherently not scalable. In principle, direct growth of van der Waals heterostructures can overcome these limitations. However, due to energetic considerations, control over the twist angle, and therefore the moiré lattice, has not yet been achieved. Herein, using TMDCs as a model system, we demonstrate the scalable growth of moiré heterostructures with continuously tunable periodicity. By controlling the substitutional alloying of larger atoms within the same group of the periodic table (S, Se), we precisely engineer the lattice parameters of the 2D layers and the moiré period. The grown moiré heterostructures have lateral size up to 200 μm and are shown to host moiré excitons. These results lay the groundwork for the integration of moiré heterostructures in emerging quantum technologies.[1] Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Nature 2018, 556 (7699), 43-50.[2] Cao, Y., Fatemi, V., Demir, A., Fang, S., Tomarken, S. L., Luo, J. Y., ... & Jarillo-Herrero, P. 2018. Nature, 556 (7699), 80-84.[3] Yu, H.; Liu, G.-B.; Tang, J.; Xu, X.; Yao, W. (2017). Science Advances 2017. 3 (11), e1701696.[4] Seyler, K. L.; Rivera, P.; Yu, H.; Wilson, N. P.; Ray, E. L.; Mandrus, D. G.; Yan, J.; Yao, W.; Xu, X. Nature 2019, 567 (7746), 66.[5] Jin, C.; Regan, E. C.; Yan, A.; Utama, M. I. B.; Wang, D.; Zhao, S.; Qin, Y.; Yang, S.; Zheng, Z.; Shi, S.; et al. Nature 2019, 567 (7746), 76.