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The layered two-dimensional (2D) materials with extraordinary optical properties play important roles in the development of ultrafast photonics, in which mode-locking lasers with a high fundamental repetition rate (>1 GHz) are of particular interest. The nonlinear optical properties of one of the emerging 2D materials, rhenium diselenide (ReSe2), have been investigated for the first time. Broadband ultrafast saturable absorption of ReSe2 from the visible to the near infrared wavelength regimes has been observed, which enables potential applications in ultrafast lasing. With typical end-pump arrangement, continuous-wave mode-locking based on the ReSe2 saturable absorber has been realized, reaching a fundamental repetition-rate of 6.5 GHz and pulse duration as short as 29 ps at 1 μm in a monolithic waveguide platform. This work indicates intriguing applications of ReSe2 for the development of on-chip ultrafast photonic devices.