Bethlehem is a city in Northampton and Lehigh counties in the Lehigh Valley region of eastern Pennsylvania, United States. As of the 2020 census, Bethlehem had a total population of 75,781, making it the second-largest city in the Lehigh Valley after Allentown and the seventh-largest city in the state. Among its total population as of 2020, 55,639 were in Northampton County and 19,343 were in Lehigh County. The city is located along the Lehigh River, a 109-mile-long (175 km) tributary of the Delaware River.

Bethlehem lies in the geographic center of the Lehigh Valley, a metropolitan region of 731 sq mi (1,890 km²) with a population of 861,899 people as of the 2020 census that is Pennsylvania's third-most populous metropolitan area and the 68th-most populated metropolitan area in the U.S. Bethlehem borders Allentown to its west and is 48 miles (77 km) north of Philadelphia and 72 miles (116 km) west of New York City.

There are four sections to the city: central Bethlehem, the south side, the east side, and the west side. Each of these sections blossomed at different times in the city's development and each contains areas recognized under the National Register of Historic Places. Norfolk Southern Railway's Lehigh Line, formerly the main line of the Lehigh Valley Railroad, runs through Bethlehem heading east to Easton and across the Delaware River to Phillipsburg, New Jersey. The Norfolk Southern Railway's Reading Line runs through Bethlehem and west to Allentown and Reading.

Bethlehem has a long historical relationship with the celebration of Christmas. The city was christened as Bethlehem on Christmas Eve 1741 by Nicolaus Zinzendorf, a Moravian bishop. In 1747, Bethlehem was the first U.S. city to feature a decorated Christmas tree. On December 7, 1937, at a grand ceremony during the Great Depression, the city adopted the nickname Christmas City USA in a large ceremony. It is one of several Lehigh Valley locations, including Egypt, Emmaus, Jordan Creek, and Nazareth, whose names were inspired by locations in the Bible.

Prior to European settlements, the areas around present-day Bethlehem and its surrounding locales along the Delaware River and its tributaries in eastern Pennsylvania were inhabited by various indigenous peoples, including the Algonquian-speaking Lenape, which had three main divisions, known by the dialects, the Unami, Unalachtigo, and Munsee. They traded with the Dutch and then British colonialists in the Mid-Atlantic region in present-day Connecticut, Delaware, Long Island, Maryland, New Jersey, the lower Hudson Valley in New York, and other regions in Pennsylvania.

On April 2, 1741, William Allen, a wealthy Philadelphia merchant and political figure, who later founded the city of Allentown, deeded 500 acres (200 ha) along the banks of the Monocacy Creek and Lehigh River to the Moravian Church. On Christmas Eve of that year, David Nitschmann and Nicolaus Zinzendorf, leading a small group of Moravians, founded the mission community of Bethlehem at the confluence of the Monocacy and Lehigh rivers. This group included John Martin Mack, who later became bishop of the Moravians in the Virgin Islands.

They established missionary communities among the Native Americans and unchurched German-speaking Christians and named the settlement after the Biblical town Bethlehem of Judea, said to be the birthplace of Jesus. "Count Zinzendorf said, 'Brothers, how more fittingly could we call our new home than to name it in honor of the spot where the event we now commemorate took place. We will call this place Bethlehem.' And so was Bethlehem named after the birthplace of the Man of Peace."

Bethlehem was started as a typical Moravian Settlement Congregation, where the Church owned all the property. In the late 1700s, Bethlehem established grist and saw mills, known as Calvin's Mills. The historic Brethren's House, Sisters' House, Widows' House, and Congregation House, with the Old Chapel, are remnants of this period of communal living.