AbstractExisting works on relay nodes placement in two‐tiered wireless sensor networks (WSNs) assume two‐dimensional terrains and ignore completely the impact of the topography on the communication quality. In contrast, this paper addresses constrained versions of the relay nodes placement problem while adopting more realistic assumptions. To ensure the connectivity of a two‐tiered WSN deployed on a three‐dimensional terrain, the proposed approach determines the relay nodes positions by analyzing the data extracted from the terrain topography. The main idea is to consider a concrete positions constraint, by requiring the placement of relay nodes only at the terrain crest points, which have a relatively wide visibility on the terrain and are well exposed to the sun. This allows minimizing the number of relay nodes required to ensure the connectivity, reduces the impact of the topography on the wireless communication quality, and offers the opportunity to equip the relay nodes with solar panels to extend their lifetimes. Based on this idea and inspired by the One‐Step constrained Relay nodes Placement (OSRP) and Realistic constrained Relay nodes PLacement (RRPL) approaches, three algorithms, called 3D‐OSRP (3D One‐Step constrained Relay nodes Placement), 3D‐RRPL‐v1 (3D Realistic constrained Relay nodes PLacement version‐1), and 3D‐RRPL‐v2 (3D Realistic constrained Relay nodes PLacement version‐2), are designed, implemented, and evaluated by extensive simulations on realistic three‐dimensional terrains. Obtained results demonstrate the relevance of considering positions constraint.