Lake Worth Beach, previously named Lake Worth, is a city in east-central Palm Beach County, Florida, United States, located about 63 miles (101 km) north of Miami. The city's name is derived from the body of water along its eastern border known as the Lake Worth Lagoon, which was named for General William J. Worth, who led United States Army forces during the last part of the Second Seminole War. Lake Worth Beach is situated south of West Palm Beach, southeast of Lake Clarke Shores, east of Palm Springs, and north of Lantana, while a small section of the city also partitions the town of Palm Beach. The 2010 census recorded a population of 34,910, which increased to 42,219 in the 2020 census. Lake Worth Beach is within the Miami metropolitan area, which was home to an estimated 6,138,333 people in 2020.

While archaeological evidence indicates that the Jaega inhabited nearby areas thousands of years ago, Samuel and Fannie James, an African American couple, became the first known settlers in modern-day Lake Worth Beach in 1885, filing a homestead claim on 187 acres (76 ha). Fannie James operated the Jewell Post Office from 1889 to 1903 to serve the few residents who lived between Lantana and West Palm Beach. A land development scheme by Bryant & Greenwood in the 1910s allowed buyers to receive a parcel of land if they purchased a lot in present-day Greenacres. Consequently, the population increased from 38 in July 1912 to 308 only five months later. The town of Lake Worth was incorporated in June 1913. Its first elected mayor was James Love, a carpenter and member of the Socialist Party of America. Thereafter, Lake Worth grew rapidly during the 1920s land boom and in the decades following World War II. Residents voted to change the official name to Lake Worth Beach in 2019.

Today, Lake Worth Beach is a city featuring several historic neighborhoods, such as the National Register of Historic Places-listed College Park and Old Lucerne, while the downtown area also has dozens of buildings that are part of the Historic Old Town Commercial District. Lake Worth Beach is one of the most ethnically and racially diverse municipalities in Palm Beach County, with a large working class population compared to other coastal cities in Palm Beach County. Several cultural events are hosted annually in the city, including a street painting festival, several ethnic festivals, and Palm Beach Pride, one of the largest LGBTQIA+ pride festivals in Florida.

Native Americans migrated into Florida beginning about 12,000 years ago. While evidence near the town of Jupiter indicates local inhabitation dating back to the Paleo-Indian period, the Jaega were the first known tribe to have resided along the Florida Atlantic coast in the areas of Martin and Palm Beach counties. The remains of shell mounds sites, mostly dating back to approximately 750–1500 CE, attest to pre-Columbian Jaega inhabitation near Lake Worth Beach, including in Boynton Beach, Palm Beach, and South Palm Beach.

Among the city's first non-indigenous settlers were Samuel and Fannie James. The Jameses were an African American couple reported to be ex-slaves, known as the Black Diamonds, who settled on the shores of the Lake Worth Lagoon near the current 5th Avenue South in 1885. A stone monument dedicated to Samuel and Fannie James at the northwest corner of Lucerne Avenue (State Road 802) and J Street inaccurately uses the date 1883, due to a transcription error. The couple made a claim for their land under the Homestead Act in 1885. Their holdings, originally 187 acres (76 ha), increased over time to more than 700 acres (280 ha), including 160 acres (65 ha) of homestead land south of Lake Avenue (State Road 802) between M and F Streets; 163.3 acres (66.1 ha) in modern-day College Park, acquired from the estate of William Stephan, where Fannie ran a pineapple farm; and 160 acres (65 ha) to the south between the current Dixie and Federal highways (Route 1 and State Road 5), acquired from Swedish immigrants Olai and Sarah Gudmundsen.

The initial name of the area's first post office was Jewel (sometimes spelled Jewell), which served the few residences between Lantana and West Palm Beach. Between 1889 and 1903, Fannie James served as postmaster of the post office, located in a small dry goods shop, which the couple operated to serve the lake traffic that connected the small pioneer homesteads located along the banks of the Lake Worth Lagoon. Area pioneers also reported that Jewell was included as a stop on the route of the barefoot mailman via the Celestial Railroad by July 1889.

Beginning in the 1890s, the Jameses sold off most of their acreage in a number of parcels ranging in size from 5 to 20 acres (2.0 to 8.1 ha) to new residents and investors. After Samuel's death in 1909, Fannie sold her remaining 156 acres (63 ha) to the developer, Palm Beach Farms Company, keeping only a 1.25 acre (0.51 ha) farmette, which lay outside the new city limits as required by the segregation provisions of the 1913 town of Lake Worth charter.

After Henry Flagler extended the Florida East Coast Railway (FEC) south from West Palm Beach to Miami in 1896, a land development scheme was created to plant a townsite between the railroad and the lake. Purchasers of agricultural lots, most of which were located in modern-day Greenacres, would also receive a small 25 by 25 ft (7.6 by 7.6 m) lot within the city of Lake Worth. The developer, Bryant & Greenwood, promoted the area to markets across the United States and Canada. They proposed to name the town Lucerne, but the United States Postal Service refused to accept the name because there already was a Lucerne post office, now a neighborhood in Miami Gardens. Therefore, the city founders changed the new town's name to Lake Worth.