Lead-free perovskite ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ manifests great potential in developing high-performance, environmentally friendly, solution-processable photodetectors (PDs). However, due to the relatively large energy bandgap, the spectrum responses of ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ PDs are limited to the ultraviolet and visible region with wavelengths shorter than 560 nm. In this work, a broadband ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ PD covering the ultraviolet, visible, and near infrared (NIR) range is demonstrated by incorporating titanium nitride (TiN) nanoparticles that are prepared with the assistance of self-assembled polystyrene sphere array. In addition, an atomically thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer is introduced at the interface between the ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ film and TiN nanoparticles to alleviate the dark current deterioration caused by nanoparticle incorporation. As a result, beyond the spectrum range where ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ absorbs light, the external quantum efficiency (EQE) of the TiN nanoparticle incorporated ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ PD is enhanced significantly compared with that of the control, displaying enhancement factors as high as 2000 over a broadband NIR wavelength range. The demonstrated enhancement in EQE arises from the photocurrent contribution of plasmonic hot holes injected from TiN nanoparticles into ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$. This work promotes the development of broadband solution-processable perovskite PDs, providing a promising strategy for realizing photodetection in the NIR region.