Formaldehyde is a strong-smelling, colorless, excitant gas that is carcinogenic to humans. It is frequently used in the interior of decorative materials such as wood paneling and carpets. Several epidemiological studies have shown that the increasing incidence of nasal and lung cancer is due to exposure to environmental formaldehyde. Thus, the rapid detection of formaldehyde with high sensitivity is vitally important for environmental monitoring and clinical diagnosis. Surface-enhanced Raman scattering (SERS) is an analytical technique that can provide fingerprint information on target materials with a sensitivity even down to the single-molecule level. However, formaldehyde molecules have a low cross section for Raman scattering and extremely weak analyte-metal interactions, and are thus scarcely detectable with conventional SERS technology. In this paper, a porous zeolitic imidazolate framework-8 (ZIF-8) shell layer was grown in situ on gold nanostars (AuNSs) for capturing and altering the route taken by formaldehyde molecules. Compared with current SERS detection methods, these as-synthesized core-shell AuNS@ZIF-8 nanocomposites with special apertures can cause formaldehyde molecules to pass through the ZIF-8 channels to the metal surface. With the help of ZIF-8 shell layers, the SERS-active AuNS@ZIF-8 substrates display extremely high sensitivity to formaldehyde molecules, with the lowest detection level almost at parts per billion (ppb). This study may therefore provide the basis for a reliable SERS strategy for detecting small molecules, especially gas samples.