Mossyrock Dam is a concrete arch-gravity dam on the Cowlitz River near Mossyrock in Lewis County, Washington, United States. The reservoir created by the dam is called Riffe Lake. The primary purpose of the dam is hydroelectric production while flood control is a secondary function. The dam is the tallest in Washington state and its hydroelectric power station supplies 40% of Tacoma Power's electricity.

Planning for the Mossyrock Dam began in the 1940s but opposition from local fishers and Washington State's Fish and Wildlife Department delayed construction. During World War II, the city of Tacoma, Washington, purchased its electricity from the Bonneville Power Administration and from Seattle, with costs of up to $1 million/year. To generate its own electricity, Tacoma City Light (now Tacoma Power) built several dams, including the Mossyrock Dam. Plans for the Mossyrock Dam were announced in 1948 but met stiff opposition. The Washington State Legislature enacted a fish sanctuary on the Cowlitz River that initially blocked the project. Tacoma City sued, with its suit being raised to the US Supreme Court a total of three times. After that, construction of the dam was approved. Construction began in 1965 and ended in 1968. On October 13, 1968, the dam's power plant generated its first electricity. Several towns were forced to evacuate ahead of the rising dam waters including Riffe, Nesika, and Kosmos.

Tacoma City Light originally proposed naming the facility the "Homer T. Bone Dam", in honor of Senator Homer Bone of Tacoma.

The community was first known as Baugh when the town was located 2.5 miles (4.0 km) farther up the Cowlitz River. Eventually moved to the area which was flooded due to the dam, it was renamed in honor of Floyd Riffe approximately in 1897.

The Mossyrock Dam's power plant contains two Francis turbine hydroelectric generators, Unit #51 is rated at 210MW and Unit #52 at 185mw. MW, or 395 MW total capacity. They are fed by two of the three installed penstocks ranging from 248 – 285 feet in overall length. The third penstock is presently unused, awaiting possible future installation of a third turbine.

In 2006, GE Energy was selected to upgrade the two units. The $50 million project included replacing the turbines, stators, wicket gates and transformers. GE sold off their rebuild contract to Andritz AG removing themselves from their contract. Hydro Installation of the first generator began in 2008; it began operation in 2009. Replacement of the second generator was completed in late 2010. The power plant upgrade increased the total capacity by 70 MW, with an expected overall energy conversion efficiency of near 95 percent.

• List of dams in the Columbia River watershed

• Tacoma News Tribune video of the Mossyrock Dam's power plant rebuild