In the recent past, employing wideband signals for underwater 3D acoustic imaging has been gaining momentum, which makes application of the existing frequency domain beamforming algorithms infeasible and complex. One potential solution is to employ the time domain delay and sum beamforming which is a popular technique used in medical ultrasound imaging, but rarely used for underwater acoustic 3D imaging applications due to the high computational complexity. In this paper, a fast and an efficient beamforming algorithm in the time domain is attempted for the reconstruction of a 3D image. The proposed method makes use of parallel 2D time domain delay and sum beamforming for the reconstruction of a 3D underwater object, followed by averaging and interpolation to reconstruct the 3D volume. Since the method adopts dynamic focusing, it is supposed to give better resolution and contrast 2D slices and hence the higher resolution 3D volume. The computational complexity and memory requirements have been reduced significantly as compared with that of conventional time-domain 3D beamforming. The reduction factor depends upon the total number of sensors used in the uniform 2D receiving array. © 2022 IEEE.