This study reports the observation of 2D-magnesium-intercalated gallium nitride (GaN) superlattices formed spontaneously by annealing a metallic Mg film on GaN at atmospheric pressure. The intercalation results in the insertion of single atomic layers of Mg between GaN monolayers, creating a unique two-dimensional (2D) Mg doping structure. The process is characterized as interstitial intercalation, leading to substantial uniaxial compressive strain perpendicular to the Mg layers. This strain induces an exceptional elastic strain exceeding -10% in GaN, one of the highest recorded for thin-film materials. The strain alters the electronic band structure and enhances hole transport along the compression direction. Additionally, the Mg sheets induce a periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics provide new insights into semiconductor doping and conductivity enhancement, as well as elastic strain engineering of nanomaterials and metal-semiconductor superlattices. The interplay between GaN and Mg has been studied since the first demonstration of p-type GaN through Mg doping. However, the low hole mobility remains a fundamental limit in III-nitride semiconductors. Applying strain to modify the band structure of GaN has been identified as a strategy to enhance carrier mobility. However, achieving and maintaining high elastic strain in GaN has been challenging. Intercalation is an important nanotechnology for fabricating artificial layered structures and has found applications in various fields. In general, van der Waals materials are chosen as hosts for intercalation due to their ability to allow the insertion of foreign atom, ion, and molecule sheets without causing excessive strain. Similarly, intercalating atomic sheets into single crystals with strong ionic and covalent bonds, such as wide-bandgap semiconductor materials, is considered extremely difficult. The study demonstrates that the spontaneous intercalation of monoatomic Mg sheets into hexagonal GaN forms a superlattice structure, a process known as 2D-Mg doping. High-resolution imaging reveals the intricate details of the Mg-intercalated GaN superlattices, confirming that the intercalant sheets consist of a single atomic layer. The interstitial occupancy of the Mg layer results in an ABCAB registry with the adjacent hexagonal GaN layers following an ABAB stacking sequence. Each Mg atom is surrounded by six N atoms, occupying an octahedral interstitial site. The Mg layer tends to repel Ga, leading to the nearest GaN monolayers exhibiting opposite polarity when positioned below and above the Mg layer. The Mg sheets induce a unique periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics offer fresh insights into semiconductor doping and conductivity enhancement, as well as elastic strain engineering of nanomaterials and metal-semiconductor superlattices. The study also highlights the perfect lattice match between GaN and Mg, which greatly reduces the mismatch strain andThis study reports the observation of 2D-magnesium-intercalated gallium nitride (GaN) superlattices formed spontaneously by annealing a metallic Mg film on GaN at atmospheric pressure. The intercalation results in the insertion of single atomic layers of Mg between GaN monolayers, creating a unique two-dimensional (2D) Mg doping structure. The process is characterized as interstitial intercalation, leading to substantial uniaxial compressive strain perpendicular to the Mg layers. This strain induces an exceptional elastic strain exceeding -10% in GaN, one of the highest recorded for thin-film materials. The strain alters the electronic band structure and enhances hole transport along the compression direction. Additionally, the Mg sheets induce a periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics provide new insights into semiconductor doping and conductivity enhancement, as well as elastic strain engineering of nanomaterials and metal-semiconductor superlattices. The interplay between GaN and Mg has been studied since the first demonstration of p-type GaN through Mg doping. However, the low hole mobility remains a fundamental limit in III-nitride semiconductors. Applying strain to modify the band structure of GaN has been identified as a strategy to enhance carrier mobility. However, achieving and maintaining high elastic strain in GaN has been challenging. Intercalation is an important nanotechnology for fabricating artificial layered structures and has found applications in various fields. In general, van der Waals materials are chosen as hosts for intercalation due to their ability to allow the insertion of foreign atom, ion, and molecule sheets without causing excessive strain. Similarly, intercalating atomic sheets into single crystals with strong ionic and covalent bonds, such as wide-bandgap semiconductor materials, is considered extremely difficult. The study demonstrates that the spontaneous intercalation of monoatomic Mg sheets into hexagonal GaN forms a superlattice structure, a process known as 2D-Mg doping. High-resolution imaging reveals the intricate details of the Mg-intercalated GaN superlattices, confirming that the intercalant sheets consist of a single atomic layer. The interstitial occupancy of the Mg layer results in an ABCAB registry with the adjacent hexagonal GaN layers following an ABAB stacking sequence. Each Mg atom is surrounded by six N atoms, occupying an octahedral interstitial site. The Mg layer tends to repel Ga, leading to the nearest GaN monolayers exhibiting opposite polarity when positioned below and above the Mg layer. The Mg sheets induce a unique periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics offer fresh insights into semiconductor doping and conductivity enhancement, as well as elastic strain engineering of nanomaterials and metal-semiconductor superlattices. The study also highlights the perfect lattice match between GaN and Mg, which greatly reduces the mismatch strain and