Maraging steel exhibits ultrahigh strength-toughness combination and is widely used in aerospace, navigation and energy sector. We elucidate here the effect of copper on the strength and toughness behavior of laser-welded T-250 maraging steel joint. The addition of Cu in the weld metal introduces epsilon-Cu precipitates during the aging period, which is beneficial to strength. Meanwhile, Cu changes the kinetics of diffusion by enhancing diffusion of Ni, resulting in the decrease of reverted austenite at grain boundaries, which has an adverse effect on toughness. Furthermore, Cu provides driving force of ferrite-austenite (α→γ) phase transformation, by increasing reverted austenite in the matrix that is beneficial to toughness. An excellent combination of high strength and high toughness of laser-welded maraging steel joint was optimized by tuning the copper content and aging temperature.