Diamond Bar is a city in eastern Los Angeles County, California, United States. The population was 55,544 at the 2010 census, down from 56,287 at the 2000 census. It is named after the "diamond over a bar" branding iron registered in 1918 by ranch owner Frederick E. Lewis.

Located at the junction of the Pomona and Orange freeways, Diamond Bar is primarily residential with shopping centers interspersed throughout the city. It is surrounded by the suburban cities, such as Brea, Walnut, Chino Hills, and Rowland Heights. The city features a public Los Angeles County golf course.

Diamond Bar has the first hydrogen fueling station to be built in Southern California, near the South Coast Air Quality Management District (AQMD) building.

In 1840, Jose de la Luz Linares received the 4,340-acre (1,760 ha) Mexican land grant Rancho Los Nogales (Ranch of the Walnut Trees) from Governor Juan Alvarado. The land grant included Brea Canyon and the eastern Walnut Valley. Linares died in 1847, and his widow sold a part of the ranch to Ricardo Vejar for $100 in merchandise, 100 calves, and the assumption of her late husband's debts. Vejar also owned the Rancho San Jose to the east, and acquired the rest of Rancho Nogales over the next 10 years.

Alexandra Patricia "Alex" Morgan (born July 2, 1989) is an American soccer player from Diamond Bar, California. She is a forward for Seattle Sounders Women and member of the US Women's National Team. Morgan was the youngest player on the USA's roster at the 2011 FIFA Women's World Cup.

Morgan was born in Diamond Bar, California, the daughter of Pamela S. (née Jeske) and Michael T. Morgan. She attended Diamond Bar High School. As a freshman at the University of California, Berkeley, Morgan was the leading scorer for the Golden Bears with eight goals in 2007. Her last goal of her freshman season came against Stanford in the second round of the 2007 NCAA Tournament, tying the game at 1–1 with less than two minutes left in regulation time and forcing the Cardinal into overtime and then to penalty kicks. However, Stanford prevailed, and went on to the third round of the tournament.

Morgan continued to lead the Golden Bears' scoring during her sophomore season with nine goals in 2008, despite being out for part of the season due to national team commitments. The Golden Bears bowed out of the first round of the 2008 NCAA Tournament while Morgan was playing at the 2008 FIFA U-20 Women's World Cup.