We report the first demonstration of a unidirectional, isolator-free 2-mmthulium-doped fiber (TDF) laser, relying on the properties of the theta cavity (ring resonator with S-shaped feedback). The core pumped theta cavity TDF laser provides sub-Watt output power with a slope efficiency of 25%, a 2 dB flat tuning range of 1900–2050 nm, and a linewidth of 0.2 nm, and achieves the extinction ratio of 18– 25 dB (depending on the feedback value) between the favored and suppressed lasing directions. It is shown that these characteristics are competitive with, if not superior to, those of conventional ring cavities. The simulation results of the linear and Kerr-nonlinear theta cavities are also presented, explaining certain unexpected features of the laser behavior and establishing the importance of the doped fiber nonlinearity on the spectral shaping of the emitted signal.