This paper reports a 32-unit phase-locked dense heterodyne receiver array at f_RF = 240 GHz. To synthesize a large receiving aperture without large sidelobe response, this chip has the following two features. The first feature is the small size of the heterodyne receiver unit, which is only λ(f_RF)/4 × λ(f_RF)/2. It allows for the integration of two interleaved 4 × 4 arrays within a 1.2 mm² die area for concurrent steering of two independent beams. Such unit compactness is enabled by the multi-functionality of the receiver structure, which simultaneously accomplishes local oscillator (LO) generation, inter-unit LO synchronization, input wave coupling, and frequency downconversion. The second feature is the high scalability of the array, which is based on a strongly coupled 2-D LO network. Large array size is realizable simply by tiling more receiver units. With the upscaling of the array, our de-centralized design, contrary to its prior centralized counterparts, offers invariant conversion loss and lower LO phase noise. Meanwhile, the entire LO network is also locked to a 75-MHz reference, facilitating phase-coherent pairing with external sub-terahertz transmitters. A chip prototype using a bulk 65-nm CMOS technology is implemented, with a dc power of 980 mW. Phase locking of the 240-GHz LO is achieved among all 32 units, with a measured phase noise of -84 dBc/Hz (1-MHz offset). The measured sensitivity (BW = 1 kHz) of a single unit is 58 fW. Compared to previous square-law detector arrays of comparable scale and density, this chip provides phase-sensitive detection with ~4300× sensitivity improvement.