This study explores the synthesis and application of LaOBr, a rare-earth oxyhalide, as a high-κ layered dielectric in van der Waals heterostructures. The researchers developed a high-temperature flux growth method to produce large, stoichiometric crystals of LaOBr, which exhibit a static dielectric constant of 9 and a wide bandgap of 5.3 eV. LaOBr was successfully exfoliated into 2D flakes and used in van der Waals heterostructure devices, demonstrating its potential as a versatile high-κ dielectric. The material showed low leakage currents, robust dielectric breakdown, and excellent performance in field-effect transistors with few-layer MoS2, achieving a subthreshold slope of ∼85 mV/dec and a low interface defect concentration of ∼1.06 × 10^12 cm^-2 eV^-1. Additionally, LaOBr was used as an encapsulation dielectric for excitonic devices based on 2D materials, enabling the modulation of excitonic species in MoSe2. These findings highlight the potential of LaOBr in advancing the field of 2D heterostructures and their applications in electronic and optoelectronic devices.This study explores the synthesis and application of LaOBr, a rare-earth oxyhalide, as a high-κ layered dielectric in van der Waals heterostructures. The researchers developed a high-temperature flux growth method to produce large, stoichiometric crystals of LaOBr, which exhibit a static dielectric constant of 9 and a wide bandgap of 5.3 eV. LaOBr was successfully exfoliated into 2D flakes and used in van der Waals heterostructure devices, demonstrating its potential as a versatile high-κ dielectric. The material showed low leakage currents, robust dielectric breakdown, and excellent performance in field-effect transistors with few-layer MoS2, achieving a subthreshold slope of ∼85 mV/dec and a low interface defect concentration of ∼1.06 × 10^12 cm^-2 eV^-1. Additionally, LaOBr was used as an encapsulation dielectric for excitonic devices based on 2D materials, enabling the modulation of excitonic species in MoSe2. These findings highlight the potential of LaOBr in advancing the field of 2D heterostructures and their applications in electronic and optoelectronic devices.