We demonstrate the application of MXenes, metallic 2D materials of transition-metal carbides, as excellent electrode materials for photonic devices. In this study, we have fabricated an InSe-based photodetector with a TiCT electrode. The photodetector with few-layer, atomically thin, TiCT (MXene) electrodes shows the avalanche carrier multiplication effect, which leads to high device performance. To improve the performance of the InSe/TiCT avalanche photodetector, we can pattern TiCT into nanoribbon arrays (a plasmonic grating structure), which enhances light absorption of the photodetector. The plasmonic InSe/TiCT avalanche photodetector exhibits low dark current (3 nA), high responsivity (1 × 10 AW), and high detectivity (7.3 × 10 Jones).