An augmented joint domain localized technique for computationally efficient polarimetric space–time adaptive processing (pSTAP) is proposed. In the proposed method, the signal vector to be detected is first estimated by using a modified least square method, and then the clutter plus noise covariance matrix (CNCM) required by pSTAP is estimated by using a newly developed dimension-reduced (DR) scheme in which the auxiliary channels in the neighbor of the target cell and the clutter ridge are jointly exploited. The inversion of the DR preprocessed CNCM estimate is finally computed via LU factorization for compensating the additional computations caused by the use of the increased auxiliary channels in the new DR procedure. Simulation results are included to illustrate the performance of the proposed method.