Mid-infrared (MIR) light sources compatible with silicon photonics are highly desirable for environmental sensing and free-space optical communications. Conventional GaSb-based quantum-well and interband-cascade lasers, however, rely on complex epitaxial heterostructures, complicating their integration with silicon photonic platforms. Here, we demonstrate a room-temperature MIR surface-emitting distributed-feedback (DFB) laser using black phosphorus (b-P) as the active gain medium. By deterministically integrating exfoliated b-P flakes onto lithographically patterned SiO₂ gratings, we obtain narrowband emission with a full width at half maximum below 3 nm under 1064 nm optical excitation. The lasing wavelength is tunable from 3.79 to 4.05 μm through the b-P flake thickness, covering a technologically important region of the mid-infrared. Room-temperature thresholds as low as (0.25 ± 0.07) mJ/cm² are achieved under nanosecond excitation. Upon cooling to 110 K, the threshold decreases tenfold to (0.015 ± 0.005) mJ/cm². These results establish planar b-P DFB cavities as a promising platform for heterogeneously integrated MIR lasers based on van der Waals semiconductors.