Newcastle is an unincorporated community and census-designated place (CDP) in Placer County, California.

According to Transcontinental Railroad Landmarks the "Regular freight and passenger trains began operating over the first 31 miles of Central Pacific's line to Newcastle June 10, 1864, when political opposition and lack of money stopped further construction during that mild winter.

[4] According to the United States Census Bureau, the CDP covers an area of 2.4 square miles (6.2 km2), 99.82% of it land, and 0.18% of it water.

186 households (34.1%) were made up of individuals, and 117 (21.5%) had someone living alone who was 65 years of age or older.

There were 597 housing units at an average density of 249.1 per square mile (96.2/km2), of which 430 (78.9%) were owner-occupied, and 115 (21.1%) were occupied by renters.