| Election Name: | 1920 United States Senate election in Arizona |
| Country: | Arizona |
| Type: | Presidential |
| Ongoing: | no |
| Previous Election: | 1914 United States Senate election in Arizona |
| Previous Year: | 1914 |
| Next Election: | 1926 United States Senate election in Arizona |
| Next Year: | 1926 |
| Election Date: | November 2, 1920 |
| Image1: | Senator Ralph Cameron.jpg |
| Nominee1: | Ralph H. Cameron |
| Party1: | Republican Party (United States) |
| Popular Vote1: | 35,893 |
| Percentage1: | 55.17% |
| Nominee2: | Marcus A. Smith |
| Party2: | Democratic Party (United States) |
| Popular Vote2: | 29,169 |
| Percentage2: | 44.83% |
| Map Size: | 205px |
| U.S. Senator | |
| Before Election: | Marcus A. Smith |
| Before Party: | Democratic Party (United States) |
| After Election: | Ralph H. Cameron |
| After Party: | Republican Party (United States) |
The 1920 United States Senate election in Arizona took place on November 2, 1920. Incumbent Democratic U.S. Senator Marcus A. Smith ran for reelection to a third term, but was defeated by former Delegate to the U.S. House of Representatives from the Arizona Territory Ralph H. Cameron in the general election. Cameron would become the first Republican elected to the office of U.S. Senator from Arizona since the state joined the union in 1912. The same year, Republican Governor Thomas Edward Campbell was reelected to a second term.
The Democratic primary took place on September 8, 1920. Incumbent U.S. Senator Marcus A. Smith was the recipient of a significant challenge in the Democratic primary, notably by Maricopa County Superior Court judge Rawghlie Clement Stanford, who would go on to become Arizona's fifth Governor in 1937. Fellow lawyer Albinus A. Worsley, as well as John W. Norton also ran in the primary. Smith narrowly staved off his primary challengers and went on to compete in the general election in November.