Interfacial interactions often promote the emergence of unusual phenomena in two-dimensional systems, including high-temperature superconductivity. Here, we report the observation of full-gap superconductivity with a maximal spectroscopic temperature up to 26 K in a BaAs monolayer grown on ferropnictide $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ (abbreviated as BFCA) epitaxial films. The superconducting gap remains robust even when the thickness of underlying BFCA is reduced to the monolayer limit, in contrast to the rapid suppression of $T_{\mathrm{c}}$ in standalone BFCA thin films. We reveal that the exceptional crystallinity of the $\mathrm{BaAs}/\mathrm{BFCA}$ heterostructures, featured by their remarkable electronic and geometric uniformities, is crucial for the emergent full-gap superconductivity with mean-field temperature dependence and pronounced bound states within magnetic vortices. Our findings open up new avenues to unravel the mysteries of unconventional superconductivity in ferropnictides and advance the development of FeAs-based heterostructures.

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• Density of states
• Multiband superconductivity
• Superconducting gap
• Vortices in superconductors
• Interfaces
• Iron-based superconductors
• Density functional theory
• Molecular beam epitaxy
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy