A thulium-doped fiber amplifier-enhanced photoacoustic spectroscopy (TDFE-PAS) sensor was developed for carbon dioxide (CO₂) detection utilizing a 2004 nm distributed feedback (DFB) laser. The thulium-doped fiber was in-band pumped by a 1567 nm source to amplify the optical power of 2004 nm to enhance the photoacoustic excitation. As a result, the photoacoustic signal was enhanced over 101 times. Based on a 7.9 mL differential PA cell, the sensor achieved a linearity of R² = 0.9997 on CO₂ detection in a wide concentration range of 0–10,000 ppm (part per million). The noise equivalent detection limit was evaluated to be 190 ppb (part per billion) at a response time of 10 s.