This paper proposes a roll-angle estimation method for the triplet towed array using the oceanic ambient noise recorded in the mid-frequency range of 1 to 10 kHz. Here, the dominance of oceanic surface-generated noises such as wind-driven and shipping noises is expected. The ratio between the approximated phase parts of the noise-coherence function regarding the triplet hydrophones is used for the roll estimation. Advantage of this technique is that explicit input regarding the oceanic environment is not required, especially for an ocean with an azimuthally uniform noise field or dominant noise sources at the end-fire direction of the horizontal array. In other cases, a roll-angle bias occurs depending on the horizontal directionality. This proposed technique is applied to ocean-ambient-noise measurements that were collected in the East Sea of Korea during August 2015. The roll angles of the triplet array are estimated from the measurements and compared with roll-sensor-measured data. The estimated roll angles agree with the measured ones within the root mean square error of 10°, and their difference tends to decrease with increasing wind speed.