Polymorph engineering in two-dimensional transition metal dichalcogenides provides an effective route for tuning structure-dependent electronic properties, yet controllable phase-selective growth remains challenging because polymorph stability is highly sensitive to growth conditions. In particular, achieving reliable phase selectivity in chemical vapor deposition (CVD) without modifying precursor chemistry or introducing external perturbations remains difficult. Here, we report a temperature-dependent CVD growth strategy for phase-selective synthesis of NbSe₂ on atomically flat WSe₂ substrates. By tuning the growth temperature, rhombohedral 3R-NbSe₂ and hexagonal 2H-NbSe₂ can be selectively obtained within distinct temperature windows, with a coexistence region observed at intermediate conditions. Structural and spectroscopic characterizations reveal clear differences in morphology, stacking configuration, and vibrational properties between the two polymorphs, while atomic-resolution microscopy confirms the formation of atomically sharp and chemically abrupt interfaces. The observed temperature-dependent phase evolution suggests that surface diffusion and nucleation behavior may play important roles during growth. In addition, metallic NbSe₂ exhibits improved carrier injection as a contact electrode, highlighting its potential for device integration. This work demonstrates a feasible approach for temperature-dependent phase-selective growth in CVD-grown two-dimensional heterostructures.