The obvious directionality inherent in interferometric synthetic aperture radar interferograms allows noise to be filtered very effectively along the fringe direction, leaving the fringe phase undamaged. This paper proposes a highly reliable method for the simultaneous estimation of the direction and density of interferograms, on the basis of which a directionally adaptive filter (DAF) is further proposed. Compared with existing filters, the DAF has a filter window whose direction is able to vary continuously with the fringe direction. The window length and width can be varied adaptively with the fringe density: a large window achieves better filtering results when the fringes are sparse, whereas a small window is better able to preserve the phase detail when they are dense. The processing results of both simulated and real data demonstrate that the DAF effectively eliminates noise and preserves detailed fringe information because its filter window performs adaptively in terms of both direction and size.