AbstractSurface‐enhanced infrared absorption (SEIRA) spectroscopy, the enhancement of molecular vibrational signals using resonant metal nanostructures, has recently attracted much attention owing to its potential use in molecular detection in chemistry, biology, and medicine. This study reports on the growth of high‐aspect‐ratio metal nanocrystals and their application for SEIRA. Highly uniform Ag nanorods are synthesized through Au nanobipyramid‐directed Ag overgrowth, and Au nanotubes are further prepared through the galvanic replacement reaction from the Ag nanorods. Both Ag nanorods and Au nanotubes possess narrow distributions in diameter and length and exhibit highly tunable plasmon wavelengths in the near and mid‐infrared regions. The Ag nanorods are employed as plasmonic nanoantennas to enhance the molecular vibration signals of cetyltrimethylammonium bromide molecules that are embedded in a mesostructured silica layer coated on the Ag nanorods. The dimension‐dependent SEIRA performances are carefully investigated both experimentally and numerically. The experimental and simulated SEIRA enhancement factors are in good agreement and both determined to be of the order ≈103. The results point out a promising route to the utilization of chemically synthesized, colloidal metal nanostructures as SEIRA platforms. The synthesis method also provides high‐quality metal nanostructures for exploring various plasmon‐based optical and sensing applications in the infrared region.