Class I terms run from the beginning of the 116th Congress on January 3, 2019, to the end of the 118th Congress on January 3, 2025. Senators in Class I were elected to office in the November 2018 general election, unless they took their seat through appointment or special election.
| Baldwin, Tammy (D-WI) |
| Brown, Sherrod (D-OH) |
| Butler, Laphonza R. (D-CA) |
| Cantwell, Maria (D-WA) |
| Cardin, Benjamin L. (D-MD) |
| Carper, Thomas R. (D-DE) |
| Casey, Robert P., Jr. (D-PA) |
| Gillibrand, Kirsten E. (D-NY) |
| Heinrich, Martin (D-NM) |
| Helmy, George S. (D-NJ) |
| Hirono, Mazie K. (D-HI) |
| Kaine, Tim (D-VA) |
| Klobuchar, Amy (D-MN) |
| Murphy, Christopher (D-CT) |
| Rosen, Jacky (D-NV) |
| Stabenow, Debbie (D-MI) |
| Tester, Jon (D-MT) |
| Warren, Elizabeth (D-MA) |
| Whitehouse, Sheldon (D-RI) |
| Barrasso, John (R-WY) |
| Blackburn, Marsha (R-TN) |
| Braun, Mike (R-IN) |
| Cramer, Kevin (R-ND) |
| Cruz, Ted (R-TX) |
| Fischer, Deb (R-NE) |
| Hawley, Josh (R-MO) |
| Romney, Mitt (R-UT) |
| Scott, Rick (R-FL) |
| Wicker, Roger F. (R-MS) |
|
Independents |
| King, Angus S., Jr. (I-ME) |
| Manchin, Joe, III (I-WV) |
| Sanders, Bernard (I-VT) |
| Sinema, Kyrsten (I-AZ) |
Article I, section 3 of the U.S. Constitution mandates the Senate's class structure.
Amendment XVII (1913) provides for direct election of Senators.
