<p>Nonlinear inerter technology, which generates nonlinear inertial forces that depend on relative acceleration, has gained significant attention for its enhanced vibration control performance compared to classic linear inerters. However, the fluid-based inerter, a cost-effective branch of inerter technology, has not yet been utilized to generate nonlinear inertia. This study introduces a fluid nonlinear inerter (FNI) capable of producing piecewise nonlinear inertial forces. The working principle and configuration of the FNI are described. A mechanical model is developed to characterize its behavior by accounting for variable inertance and coupled damping effects. An FNI prototype is fabricated, and experimental tests are conducted to verify the proposed mechanical model. Subsequently, the FNI is integrated into a vibration isolator to assess its performance enhancement. Analytical solutions of the isolator with FNI are derived using the averaging method and verified through numerical integration method. Performance assessments of the isolator with FNI are carried out for various inertance parameters. Comparative analyses are conducted between the linear isolator, the isolator with classic fluid inerter (FI), and the isolator with FNI, focusing on three key performance indices: peak dynamic displacement amplitude, peak force transmissibility and effective isolation frequency band. The experimental results show good agreement with the theoretical predictions, confirming the accuracy of the proposed mechanical model of FNI. The vibration isolator equipped with FNI exhibits lower peak force transmissibility and a broader effective isolation band compared to both the linear isolator and the isolator with classic FI, highlighting the superiority of the proposed FNI. The FNI is concluded to be a promising candidate for nonlinear inerter technology, offering significant potential for improving the performance of vibration isolators.</p>