It is challenging to apply laser metal deposition (LMD) in underwater environment to realize on-site repair of marine equipment due to the potential impact of water. In the present work, we report an innovative underwater repair technique termed underwater laser metal deposition (ULMD) which can overcome the challenges. This new technique renders both technical and theoretical advancements from the following aspects: (1) A special in-house designed drainage nozzle was integrated with the laser cladding head to create local dry cavity which ensured the successful manufacturing of titanium alloy Ti-6Al-4V in underwater environment; (2) Unique microstructure formation/evolution mechanisms have been revealed for the ULMD process, which significantly differ from those of the in-air LMD process; (3) The hydrogen content has been well controlled during ULMD, which can effectively prevent the formation of hydrogen-induced cracks; (4) The mechanical properties of the ULMD Ti-6Al-4V parts were equal or even better than that fabricated by in-air LMD or SLM technique. In the meantime, a systematically parametric study was performed for this new technique and the experimental results showed that a high laser power (1600 W), a reasonable scanning velocity (800 mm/min) and a cross-hatching strategy were essential for achieving minimum metallurgical defects and full densification, which can provide very helpful guidance for the future research in this area. This research work opens a new avenue which makes underwater 3D-printing an applicable tool when coping with the fabrication and repair of customized components or complex shape parts in underwater environment. •Powder feeding underwater laser metal deposition was firstly carried out on Ti-6Al-4V.•The quality of Ti-6Al-4V alloy by ULMD was equal or even better than that by in-air LMD.•How the underwater environment affects metallurgical behavior of the melt pool was studied.•Unique microstructure formation/evolution mechanisms were related with ULMD thermal process.•Influence mechanisms of the ULMD microstructure on tensile properties were revealed.