Tunicate (marine invertebrates)-derived cellulose nanocrystals (T-CNC) possess unique structural and physicochemical properties compared to other wood-based CNCs. This study aimed to characterize and utilize T-CNC as a stabilizer in Pickering emulsion (PE), highlighting a sustainable alternative to conventional surfactant-based emulsifiers. Characterization of T-CNC revealed a rod-shaped morphology with dimensions of 1694 ± 925 nm in length and 13 ± 3 nm in width, resulting in an aspect ratio of 122 ± 45, and high crystallinity (87.6%). Its zeta potential ranged from −4.4 to −45.5 mV across pH 2–10 and contact angles <50° indicate strong water wettability. T-CNC at 0.2%, 0.3%, and 0.4% (w/w) at pH 3 and 5 was used to prepare 20 wt% oil-in-water PE using a high-shear homogenizer followed by ultrasonication. Ultrasonication significantly improved the emulsion stability compared to only high-shear homogenization, decreasing droplet size by 31.4–50.8% and 55.7–89.3% for pH 3 and pH 5, respectively. PEs developed at pH 3 demonstrated smaller droplet sizes, better stability with minimal coalescence after 7 days, and enhanced gel-like rheological behaviour compared to PEs at pH 5, which displayed flocculation and coalescence. The gel strength of the pH 3 PEs increased with T-CNC concentration, as evidenced by progressively denser droplet packing, consistent with stronger interfacial anchoring (higher detachment energy) and reduced coalescence. This study underscores T-CNC’s superior efficiency in stabilizing PEs at low concentrations, offering a green, high-performance solution for food, cosmetic, and pharmaceutical applications.