Lacey is a city in Thurston County, Washington, United States. It is a suburb of Olympia with a population of 53,526 at the 2020 census, making it the 24th most populous city in Washington. Lacey is located along Interstate 5 between Olympia and the Nisqually River, which marks the border with Pierce County and Joint Base Lewis–McChord.

Lacey was originally called Woodland after settlers Isaac and Catherine Wood, who claimed land there in 1853. By 1891, the railroad had come to the community of Woodland and the residents decided it was time to apply for a post office. The request was denied because there was already a town called Woodland on the Columbia River. The name Lacey was chosen for the new post office application, presumably after O. C. Lacey, a Justice of the Peace in Olympia. The small settlements of Woodland and Chambers Prairie consolidated into Lacey in the 1950s. The city of Lacey was not officially incorporated until December 5, 1966. At the time, the main industries were cattle, milk, forest products, and retail. Lacey became a commuter town for Olympia, Fort Lewis and to some extent, Tacoma; in recent years, however, business developments, community groups, and population growth have led Lacey to develop into a city in its own right.

Lacey generally lies between Olympia to the west and the Nisqually River delta (which includes the Billy Frank Jr. Nisqually National Wildlife Refuge) to the east. The city's northernmost point is along the Nisqually Reach on Puget Sound adjacent to Tolmie State Park and its southern boundary is a section of the BNSF Railway's Seattle Subdivision, a major freight and passenger railway.

According to the United States Census Bureau, the city has a total area of 16.51 square miles (42.76 km2), of which, 16.06 square miles (41.60 km2) is land and 0.45 square miles (1.17 km2) is water.

As of the 2020 census, there were 53,526 people, 20,984 households in the city.

As of the 2010 census, there were 42,393 people, 16,949 households, and 10,869 families residing in the city. The population density was 2,639.7 inhabitants per square mile (1,019.2/km2). There were 18,493 housing units at an average density of 1,151.5 per square mile (444.6/km2). The racial makeup of the city was 74.2% White, 5.4% African American, 1.2% Native American, 8.0% Asian, 1.7% Pacific Islander, 2.6% from other races, and 7.0% from two or more races. Hispanic or Latino of any race were 9.2% of the population.

There were 16,949 households, of which 33.9% had children under the age of 18 living with them, 46.7% were married couples living together, 12.9% had a female householder with no husband present, 4.5% had a male householder with no wife present, and 35.9% were non-families. 28.3% of all households were made up of individuals, and 11% had someone living alone who was 65 years of age or older. The average household size was 2.44 and the average family size was 2.99.

The median age in the city was 34 years. 24.6% of residents were under the age of 18; 10.1% were between the ages of 18 and 24; 29.5% were from 25 to 44; 21.8% were from 45 to 64; and 14.1% were 65 years of age or older. The gender makeup of the city was 47.4% male and 52.6% female.

As of 2000 the median income for a household in the city was $46,848, and the median income for a family was $54,923. Males had a median income of $41,053 versus $32,497 for females. The per capita income for the city was $20,224. About 7.1% of families and 8.8% of the population were below the poverty line, including 10.5% of those under age 18 and 5.5% of those age 65 or over.

Mushroom Corner is located along Interstate 5 and is included in the Tanglewilde-Thompson Place Census-designated place (CDP) for Census purposes. The "corner" in Mushroom Corner is located at the intersection of Steilacoom Road SE and Marvin Road SE. The community takes its name from the local mushroom crop; the Ostrom Mushroom Farm that formerly operated nearby.

South Sound Center is a shopping mall in Lacey.

Lacey was the twelfth city to be designated an official "Green Power Community" by the U.S. Environmental Protection Agency for its use of renewable energy sources; 8.4% of its total energy use comes from green power sources. It is working to meet its Alternative Energy Initiative, which includes using 100 percent green electrical energy in all of its municipal buildings, parks, utilities, and 3,000 streetlights and traffic signals; providing electric vehicle charging stations to visitors and employees at its city hall and library campus; and initiating conversion of its municipal fleet to energy efficient vehicles powered by electricity, hybrid technology, and 80/20 biofuel. Lacey has received the "Tree City, USA" designation from the National Arbor Day Foundation for twenty six years.

The city is entirely within the boundaries of North Thurston Public Schools, the largest school district in Thurston County. Lacey is also home to various faith based schools, such as Holy Family School (Roman Catholic Preschool through 8th grade), Faith Lutheran School (Preschool through 8th Grade) and Foundation Campus, which includes Community Christian Academy (Pre-school to Middle School) and Northwest Christian High School. Lacey is also the home of Pope John Paul II High School.

Lacey is also home to Saint Martin's University, a private four-year university that was founded in 1895 by the Order of Saint Benedict.

Lacey is bisected from west to east by Interstate 5, a north–south freeway connecting Seattle to Portland, Oregon. The first diverging diamond interchange in Washington opened in August 2020 at a junction in Lacey between Interstate 5 and Marvin Road (State Route 510).

The city is served by Intercity Transit, the public transit system for Thurston County, and is the eastern terminus of The One bus rapid transit line. The agency also runs express buses from Olympia and Lacey to Tacoma with onward connections to other transit systems. The county's only Amtrak station, Centennial Station, is located near Lacey's southern boundary and is served by daily Cascades and Coast Starlight trains.