The meridian 120° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, North America, the Pacific Ocean, the Southern Ocean, and Antarctica to the South Pole. The 120th meridian west forms a great circle with the 60th meridian east. In Canada the border between British Columbia and Alberta is defined by the meridian north of where it intersects the Continental Divide of the Americas, and in the United States part of the border between California and Nevada follows it. The mean solar time at this meridian determines the time for the Pacific Time Zone (UTC−08:00) during standard time. Most of the year however, it is the meridian for Alaska Daylight Time, as daylight saving time is observed for two-thirds of the year.

From Pole to Pole

Starting at the North Pole and heading south to the South Pole, the 120th meridian west passes through: ! scope="col" width="130" | Co-ordinates ! scope="col" | Country, territory or sea ! scope="col" | Notes ! scope="row" style="background:#b0e0e6;" | Arctic Ocean ! scope="row" | 🇨🇦 ! scope="row" style="background:#b0e0e6;" | M'Clure Strait ! scope="row" | 🇨🇦 ! scope="row" style="background:#b0e0e6;" | Prince of Wales Strait ! scope="row" style="background:#b0e0e6;" | Amundsen Gulf ! scope="row" | 🇨🇦 ! scope="row" | 🇺🇸 ! scope="row" style="background:#b0e0e6;" | Pacific Ocean ! scope="row" | 🇺🇸 ! scope="row" style="background:#b0e0e6;" | Pacific Ocean ! scope="row" style="background:#b0e0e6;" | Southern Ocean ! scope="row" | Antarctica

Discrepancies at the California–Nevada border

When California attained statehood in 1850, it adopted 120th meridian west as its eastern border.<ref name="CA v. NV"> Between 1855 and 1900 there were six surveys to locate 120 degrees, with each locating the line of longitude differently. In 1872, Alexey W. Von Schmidt undertook the survey of the state line. He marked his survey line with stones, wood, and iron markers; the only one who placed such markers A new survey in 1893 showed that the Von Schmidt line was 1,600 to 1,800 feet west of the actual 120 degrees. However, California and Nevada both recognize the 1872 Von Schmidt survey and the 1893 survey as the state line. Google maps shows that the Verdi, Nevada California–Nevada boundary marker, located at 39.52451°N, -120.00186°W, is approximately 525 ft west of 120 degrees longitude, with a similar deviation along the length of the line from Oregon to Lake Tahoe.