Precise manipulation of optical properties through the structure‐evolution of plasmonic nanoparticles is of great interest in biomedical fields including bioimaging and phototherapy. However, previous success has been limited to fixed assembled structures or visible–NIR‐I absorption. Here, an activatable NIR‐II plasmonic theranostics system based on silica‐encapsulated self‐assembled gold nanochains (AuNCs@SiO2) for accurate tumor diagnosis and effective treatment is reported. This transformable chain structure breaks through the traditional molecular imaging window, whose absorption can be redshifted from the visible to the NIR‐II region owing to the fusion between adjacent gold nanoparticles in the restricted local space of AuNCs@SiO2 triggered by the high H2O2 level in the tumor microenvironment (TME), leading to the generation of a new string‐like structure with strong NIR‐II absorption, which is further confirmed by finite‐difference‐time‐domain (FDTD) simulation. With the TME‐activated characteristics, AuNCs@SiO2 exhibits excellent properties for photoacoustic imaging and a high photothermal conversion efficiency of 82.2% at 1064 nm leading to severe cell death and remarkable tumor growth inhibition in vivo. These prominent intelligent TME‐responsive features of AuNCs@SiO2 may open up a new avenue to explore optical regulated nano‐platform for intelligent, accurate, and noninvasive theranostics in NIR‐II window. An activatable NIR‐II plasmonic theranostics system for accurate tumor diagnosis and effective treatment is reported. This gold nanochain structure breaks through the traditional molecular imaging window, shifting from the visible to the NIR‐II region, owing to fusion between adjacent gold nanoparticles in the restricted local space triggered by the high H2O2 level in the tumor microenvironment.