Purpose: Real-time three-dimensional(3D) magnetic resonance(MR) imaging is challenging because of slow MR signal acquisition, leading to highly under-sampled k-space data. Here, we proposed a deep learning-based, k-space-driven deformable registration network(KS-RegNet) for real-time 3D MR imaging. By incorporating prior information, KS-RegNet performs a deformable image registration between a fully-sampled prior image and on-board images acquired from highly-under-sampled k-space data, to generate high-quality on-board images for real-time motion tracking. Methods: KS-RegNet is an end-to-end, unsupervised network consisting of an input data generation block, a subsequent U-Net core block, and following operations to compute data fidelity and regularization losses. The input data involved a fully-sampled, complex-valued prior image, and the k-space data of an on-board, real-time MR image(MRI). From the k-space data, under-sampled real-time MRI was reconstructed by the data generation block to input into the U-Net core. In addition, to train the U-Net core to learn the under-sampling artifacts, the k-space data of the prior image was intentionally under-sampled using the same readout trajectory as the real-time MRI, and reconstructed to serve an additional input. The U-Net core predicted a deformation vector field that deforms the prior MRI to on-board real-time MRI. To avoid adverse effects of quantifying image similarity on the artifacts-ridden images, the data fidelity loss of deformation was evaluated directly in k-space. Results: Compared with Elastix and other deep learning network architectures, KS-RegNet demonstrated better and more stable performance. The average(±s.d.) DICE coefficients of KS-RegNet on a cardiac dataset for the 5-, 9-, and 13-spoke k-space acquisitions were 0.884±0.025, 0.889±0.024, and 0.894±0.022, respectively; and the corresponding average(±s.d.) center-of-mass errors(COMEs) were 1.21±1.09, 1.29±1.22, and 1.01±0.86 mm, respectively. KS-RegNet also provided the best performance on an abdominal dataset. Conclusion: KS-RegNet allows real-time MRI generation with sub-second latency. It enables potential real-time MR-guided soft tissue tracking, tumor localization, and radiotherapy plan adaptation.