Abstract Multiplex imaging in the second near‐infrared window (NIR‐II, 1000–1700 nm) provides exciting opportunities for more precise understanding of biological processes and more accurate diagnosis of diseases by enabling real‐time acquisition of images with improved contrast and spatial resolution in deeper tissues. Today, the number of imaging agents suitable for this modality remains very scarce. In this work, we have synthesized and fully characterized, including theoretical calculations, a series of dimeric Ln III /Ga III metallacrowns bearing Ru II polypyridyl complexes, LnRu‐3 (Ln=Y III , Yb III , Nd III , Er III ). Relaxed structures of YRu‐3 in the ground and the excited electronic states have been calculated using dispersion‐corrected density functional theory methods. Detailed photophysical studies of LnRu‐3 have demonstrated that characteristic emission signals of Yb III , Nd III and Er III in the NIR‐II range can be sensitized upon excitation in the visible range through Ru II ‐centered metal‐to‐ligand charge transfer (MLCT) states. We have also showed that these NIR‐II signals are unambiguously detected in an imaging experiment using capillaries and biological tissue‐mimicking phantoms. This work opens unprecedented perspectives for NIR‐II multiplex imaging using Ln III ‐based molecular compounds.