Rare-earth-doped fiber lasers are emerging as promising high-power mid-infrared sources for the 2.6–3.0 μm and 3.3–3.8 μm regions based on erbium and holmium ions. The intermediate wavelength range, however, remains vastly underserved, despite prospects for important manufacturing and defense applications. Here, we demonstrate the potential of dysprosium-doped fiber to solve this problem, with a simple in-band pumped grating-stabilized linear cavity generating up to 1.06 W at 3.15 μm. A slope efficiency of 73% with respect to launched power (77% relative to absorbed power) is achieved—the highest value for any mid-infrared fiber laser to date, to the best of our knowledge. Opportunities for further power and efficiency scaling are also discussed.