Taking advantage of metal–semiconductor junctions, functional nanocomposites have been designed and developed as active substrates for surface-enhanced Raman scattering (SERS) sensing systems. In this work, we prepared three types of nanocomposites based on manganese oxide (MnO₂) nanostructures and electrochemically synthesized silver nanoparticles (e-AgNPs), which differed according to the morphologies of MnO₂. The SERS performance of MnO₂ nanosheet/e-Ag (MnO₂-s/e-Ag), MnO₂ nanorod/e-Ag (MnO₂-r/e-Ag), and MnO₂ nanowire/e-Ag (MnO₂-w/e-Ag) was then evaluated using tricyclazole (TCZ), a commonly used pesticide, as an analyte. Compared to the others, MnO₂-s/e-Ag exhibited the most remarkable SERS enhancement. Thanks to its large surface area and ability to accept/donate the electrons of the semiconductor, MnO₂-s acted as a bridge to improve the charge transfer efficiency from e-Ag to TCZ. In addition, the MnO₂ content of the nanocomposites was also considered to optimize the SERS sensing performance. With the optimal MnO₂ content of 25 wt%, MnO₂-s/e-Ag could achieve the best SERS performance, allowing the detection of TCZ at concentrations down to 6 × 10⁻¹² M in standard solutions and 10⁻¹¹ M in real rice samples.