Rotational transitions of the nonlinear triatomic molecular anion NH_{2}^{-} have been observed by terahertz spectroscopy in a cryogenic radio frequency ion trap. Absorption of terahertz photons has been probed by rotational state-dependent photodetachment of the trapped negative ions near the detachment threshold. Using this two-photon scheme, the two lowest rotational transitions for the asymmetric top rotor NH_{2}^{-} have been found. For the para nuclear spin configuration, the 1_{0}←0_{0} transition frequency was determined to be 933 954(2) MHz, and for the ortho configuration the 1_{+1}←1_{-1} transition frequency was determined to be 447 375(3) MHz. This result appears to preclude the recent tentative assignment of an interstellar absorption feature to NH_{2}^{-}.