Summary: Development of highly sensitive, stable, and repeatable surface-enhanced Raman scattering (SERS) substrates is crucial for analytical detection, which is a challenge for traditional metallic structures. Here, by exploiting the high surface activity of the 1T′ transition metal telluride, we fabricate high-density gold nanoparticles (AuNPs) that are prepared spontaneously in situ on the 1T′ MoTe2 atomic layers, forming a plasmonic-2D material hybrid SERS substrate. This AuNP formation is unique to the 1T′ phase, which is repressed in 2H MoTe2 with lower surface activity. The hybrid structure generates coupling effects of electromagnetic and chemical enhancements, as well as excellent molecule adsorption, leading to ultrasensitive (4 × 10−17 M) and reproducible detection. Flexible SERS tapes are demonstrated in practical applications. Our approach facilitates ultrasensitive SERS detection by a facile method, as well as an enhanced mechanistic understanding of SERS beyond plasmonic effects.