Miniaturized laser spectroscopy capable of in situ and real-time ppb-level trace gas sensing is of fundamental importance for numerous applications, including environment monitoring, industry process control, and biomedical diagnosis. Benchtop laser spectroscopy systems based on direct absorption, photoacoustic, and Raman effects exhibit high sensitivity but face challenges for in situ and real-time gas sensing due to their bulky size, slow response, and offline sampling. We demonstrate a microscale high-performance all-fiber photoacoustic spectrometer integrating the key components, i.e., the photoacoustic gas cell and the optical microphone, into a single optical fiber tip with a diameter of 125 μm. Without a long optical path to enhance the light–gas interaction, the fiber-tip gas cell with acoustic-hard boundary tightly confines and amplifies the local photoacoustic wave, compensating for the sensitivity loss during miniaturization. This localized acoustic wave is demodulated by high-sensitivity fiber-optic interferometry, enabling a ∼9 ppb detection limit for acetylene gas approaching the benchtop system. The microscale fiber spectrometer also exhibits a short response time of ∼18 ms and a subnanoliter sample volume, not only suitable for routine real-time in situ trace gas measurement but also inspiring new applications such as two-dimensional gas flow concentration mapping and in vivo intravascular blood gas monitoring as showcased.