Titanium nitride (TiN) is an ideal material for infrared plasmonics due to its excellent optical properties, high melting temperature, mechanical and chemical stabilities, and bio- and CMOS compatibilities. In this work, we demonstrate that ultrathin and scalable TiN epitaxial structures can be applied for tunable infrared plasmonics, extending into near- to mid-infrared spectral regions. The ultrathin (111)-oriented TiN epitaxial films studied here were grown on c-plane sapphire wafers without any wetting layer by ultrahigh-vacuum nitrogen-plasma-assisted molecular-beam epitaxy. This method allows for stoichiometric TiN growth without the issue of contamination (especially oxygen) in conventional TiN growth techniques. Structural analyses for these films validate their single-crystalline properties with continuous film morphologies down to a few nanometers in thickness. Furthermore, the frequency-tunable (wavelength range: 1–4 μm) plasmonic metasurfaces have been demonstrated by controlling surface plasmon resonances via lithographically patterning of ultrathin TiN epitaxial films with varying thicknesses (4–30 nm) and grating structure parameters (pitch: 300–1200 nm, width: 200–800 nm). The tunable plasmonic metasurfaces based on ultrathin TiN epitaxial films hold great promise for emerging infrared plasmonic applications, such as thermal photovoltaics requiring narrow-band emitters, photodetectors, and biosensors in the near- and mid-infrared spectral regions.