In the present research, fiber laser beam welding (FLBW) of thin nickel sheets in two different mediums, i.e., air and water, has been investigated. In air medium, the fiber laser welding operations are performed at different laser powers (60, 80 and 100 W) and scanning speeds (40, 60 and 80 mm/min). Underwater welding operations have also been investigated at three different laser powers (330, 350 and 370 W) and two different scanning speeds (20 and 40 mm/min). The results show that welding of Ni sheet with higher laser power results in prominent heat-affected zone, increase in both microhardness and wt% of oxide formation in the weldment in both air and water medium. It is found that the increase in microhardness of weldment leads to the reduction in grain size. The heat-affected zone is reduced when scanning speed is increased. The wt% of oxygen increases with an increase in power during FLBW operation at constant scanning speed, and the oxide formation can be controlled by varying the laser power and scanning speed. The welding medium has significant influences on the properties of the weldments.