William Westwood Lockyer (born May 8, 1941) is an American politician and lawyer from the state of California. A Democrat, he served in both houses of the state legislature, having been a member of the California State Assembly from 1973 to 1982 and the California State Senate from 1982 to 1998. He spent the last four years of his State Senate tenure as president pro tempore. He then served as California Attorney General from 1999 to 2007, and as California State Treasurer from 2007 to 2015.

Lockyer was born in Hayward, California, on May 8, 1941. He attended the University of California, Berkeley, graduating with a B.A. in Political Science in 1965. The following year, he received a Teaching Credential from CSU in Hayward, then worked for his father's roofing company and as a fork-lift driver at Ward's before getting his first job with the Legislature on the staff of Assemblyman Robert W. Crown. In 1986, Lockyer graduated with a J.D. from University of the Pacific McGeorge School of Law. (He returned to the classroom in 2009 as a non-tenured college professor, teaching undergraduate courses in American State Politics at the University of Southern California and the University of California at Irvine.) With his early legislative experience, Lockyer began his own political career as a School Board member of the San Leandro Unified School District, as chair of the Alameda County Democratic Central Committee, and California coordinator of Senator George McGovern's 1972 campaign for the Presidency.

Lockyer was married in April 2003 to public service attorney Nadia Lockyer, a former Alameda County Supervisor, with whom he has three children: a son born in 2004; and two twin sons, born in December 2015. By an earlier marriage, he also has an adult daughter who is an attorney at the National Aeronautics and Space Administration (NASA).

Lockyer first won a State Assembly seat in a Special Election of September 4, 1973, following the accidental death of his political mentor, Bay Area Assemblyman Robert W. Crown. He served in the legislature for the next twenty-five years, more than half that time in the state senate, where, in 1994, he was chosen by his peers to be President Pro Tem, the most powerful position of the upper legislative house.

In his spare time, Lockyer attended law school classes in Sacramento and received a Juris Doctor degree from the McGeorge School of Law, University of the Pacific.

As legislator, Lockyer won close friends on both sides of the partisan aisle, including Jim Brulte, Republican Minority Leader of the state senate, who would long remember Lockyer's skill at compromise and consensus-building., and Democratic Speaker of the Assembly Willie Brown, who recalled that, by the time Lockyer left the legislature in 1998, "Capitol insiders took his prolific effectiveness for granted."

As a freshman legislator in 1974, Lockyer wrote the first legislation to provide state funding for emergency oil spill decontamination. During his legislative career, as a close ally of environmental pressure groups like the Sierra Club and the Planning and Conservation League, he wrote other environmental laws, including the first state regulation of trucks hauling toxic substances on California roads and highways, which preceded federal policies adopted by the EPA.

Lockyer considered his greatest environmental achievement to be his 1987 bill to create a Bay Trail, which he envisioned as an eventual 500-mile-long hiking and cycling path, a continuous recreational corridor, with adjacent bayshore parks and protected natural habitats, that would entirely encircle San Francisco and San Pablo Bays. Requiring city, county and regional cooperation, the Bay Trail marked its 20th year in 2009 with 293 miles so far open to hikers, bicyclists, joggers and walkers. As nominal "father" of the Bay Trail, a segment of the shoreline had been named in Lockyer's honor.