The superconductor-insulator transition as a paradigm of quantum phase transitions has attracted tremendous interest over the past three decades. While the magnetic field and carrier density can be tuned to drive the transition, the role of disorder in the transition is not well understood due to the complicated interplay between superconductivity and electron localization. In this Letter, we controllably introduce disorder in a two-dimensional high-temperature superconductor by depositing iron clusters onto the superconducting monolayer Fe(Te,Se) crystalline film. The spectral evolution from superconducting gaps to insulating gaps with increasing disorder is detected by scanning tunneling spectroscopy measurements. When the disorder is strong, large $U$-shaped gaps are observed and attributed to the localization-enhanced Cooper pair correlation. Our observations provide insight into the emergent phases of low-dimensional and high-temperature superconductors with disorder.

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• Superconductor-insulator transition
• High-temperature superconductors
• Iron-based superconductors
• Monolayer films
• Molecular beam epitaxy
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy