This paper presents the concept of a 2D m × n waveguide-based power combiner (PC) that is scalable with respect to the operating frequency band and number of input ports. To our knowledge, this work reports the first planar (2D) power combiner is reported. The novelty of the approach relies on using H-plane rectangular waveguide T-junctions and low-loss polarization twisters in between vertically stacked T-junctions to facilitate scalability. The work is motivated by the aim to coherently combine the output power of multiple modified uni-travelling carrier (MUTC) Terahertz (THz) waveguide photodiodes (PD) in a 2D array configuration. In the manuscript, the design of a 2×2 planar waveguide power combiner for the WR3 band (220–320 GHz) is reported and it is also shown that this block can be further extended to m × n input ports. Full-wave numerical analysis of the proposed 2×2 power combiner shows a return loss of 11 dB at the output port and an average transmission coefficient of about -6.5 dB, i.e., an overall power combining efficiency of ~90%. Furthermore, to enable 2D photodiode array integration, the manuscript presents a new slot-bow tie antenna integrated MUTC photodiode for radiating the optically generated THz power from each PD vertically into the rectangular waveguide. Reflection loss and insertion loss for the slot bow-tie antenna are shown to be better than 10 dB and 1.4 dB over the full WR3 band, respectively. To prove scalability of the power combiner concept w.r.t. the number of input ports, also a 2×4 power combiner is analyzed. Results reveal a return loss better than 10 dB from 225 to 318 GHz and a transmission coefficient of approximately -9.7 dB at 300 GHz, i.e., a power combining efficiency of ~85%.