The exploration of multifunctional perovskite nanostructures for flexible sensor/energy harvesting applications is of great interest. Herein, highly crystalline ferroelectric Bi4Ti3O12 nanoparticles (NPs) were brush-coated onto flexible Kapton film without using costly instrumental techniques or polymer templates. Moreover, flexible ferroelectric high-temperature thermistors (FHTTs) having high thermal sensitivity (β(100/260) = 6515 K) and a broad working temperature range (25–260 °C) were developed based on bismuth-layered perovskites. Polarization (P) vs. Electric field (E) (P–E) loops of the orthorhombic phase of Bi4Ti3O12 NPs were measured at different frequencies under poled and unpoled conditions. The Bi4Ti3O12 NPs exhibited a multi-fold improvement of remnant polarization (Pr ≈ 6 μC/cm2) at the coercive field (Ec ≈ 90 kV/cm) over that previously reported. Lastly, the prototype of a self-powered-FHTT was fabricated, which could be used as an efficient environmental temperature warning/monitoring system free of complex battery sources. The Bi4Ti3O12 NPs/PVDF composite film-based piezoelectric nanogenerator (BPCF-PNG) used an eco-friendly power source (generating 12.7 mW/m2 at 300 MΩ upon 4 N applied force) to drive the FHTT. The brush-coating method to prepare flexible ferroelectric films is highly suitable for large-scale fabrication, and is also an eco-friendly and time-efficient process. The detailed investigation reported herein is a novel approach to next-generation ferroelectric-based sensors, memristors, and energy harvesters.