The Colorado State Senate is the upper house of the Colorado General Assembly, the state legislature of the US state of Colorado. It is composed of 35 members elected from single-member districts, with each district having a population of about 123,000 as of the 2000 census. Senators are elected to four-year terms, and are limited to two consecutive terms in office. Senators who are term-limited become eligible to run again after a one-term (four year) respite.

The Colorado Senate convenes at the State Capitol in Denver.

The first meeting of the Colorado General Assembly took place from November 1, 1876, through March 20, 1877.^[2] Lafayette Head was the first state senate president.^[2]

The lieutenant governor served as Senate President until 1974 when Article V, Section 10 of the state constitution was amended, granting the Colorado Senate the right to elect one of its own members as President.^[2] Fred Anderson was the first state senate president elected after the amendment.^[2] Ruth Stockton was the first woman to become Senate's president pro tempore, serving from 1979 to 1980.^[3][4]

The Colorado Senate has 35 members elected to staggered four-year terms. Half the chamber is elected in the same year as gubernatorial elections, with the other half elected in the same year as presidential elections.

State senators are term-limited to two consecutive terms, equivalent to eight years. Term-limited former members can run again after a four-year break. Vacancies in legislative offices are generally filled by political party vacancy committees, rather than special elections. Vacancy appointees who fill the first half of a state senator's term must stand for election at the next even year November election for the remainder of the state senate term for the seat to which the state senator was appointed.

With the notable exceptions listed below, the Colorado Senate operates in a manner quite similar to the United States Senate.^[5]

Regular sessions are held annually and begin no later than the second Wednesday in January. Regular sessions last no more than 120 days. Special sessions may be called at any time by the governor of Colorado or upon written request of two-thirds of the members of each house, but are infrequent. Some committees of the General Assembly work between sessions and have limited power to take action without General Assembly approval between legislative sessions.

Joint procedural rules of the two chambers require most legislation to be introduced very early in the legislative session each year, and to meet strict deadlines for completion of each step of the legislative process. Joint procedural rules also limit each legislator to introducing five bills per year, subject to certain exceptions for non-binding resolutions, uniform acts, interim committee bills and appropriations bills. Most members of the General Assembly decide which bills they will introduce during the legislative session (or most of them) prior to its commencement, limiting the ability of members to introduce new bills at constituent request once the legislative session has begun.

Most bills adopted by the General Assembly include a "safety clause" (i.e. a legislative declaration that the bill concerns an urgent matter) and take effect on July 1 following the legislative session unless otherwise provided. Some bills are enacted without a "safety clause" which makes it possible to petition to subject those bills to a referendum before they take effect, and have an effective date in August following the legislative session unless otherwise provided.^[5]

Colorado's legislature does not have an analog to the filibuster in the United States Senate requiring a supermajority for approval of any matter. The state lieutenant governor does not have the power to preside or break tie votes in either house of the General Assembly.^[2] New executive branch rules are reviewed annually by the legislature and the legislature routinely invalidates some of them each year.

The General Assembly does not have a role in the appointment or retention of state judges, although it must authorize the creation of each judgeship.

Many state agencies and programs are subject to "sunset review" and are automatically abolished if the General Assembly does not reauthorize them.

In 1885, the Colorado Senate appointed its first chaplain, Methodist circuit riding missionary, "Father" John Lewis Dyer.^[6]

Based on the 2010 census, each state senator represents 143,691 constituents. The 2020 Colorado Elections resulted in the Democratic Party maintaining a majority of seats in the senate. Democrats currently hold a majority in the Senate in the 73rd General Assembly: 21 Democrats and 14 Republicans.

At the 2022 elections 17 senate seats came up for re-election. As a result the composition of the State Senate at the beginning of the 74th General Assembly will likely be 23 Democrats and 12 Republicans.^[7]

With the Democratic majority, Steve Fenberg serves as President of the Senate and the Majority Leader position is currently vacant.

*Senator was originally appointed

#Senator is ineligible for re-election due to term limits

• Outline of Colorado
• Index of Colorado-related articles
• State of Colorado
  • Law and government of Colorado
    • Governor of Colorado
      • Lieutenant Governor of Colorado
    • Colorado General Assembly
      • Colorado Senate
      • Colorado House of Representatives
      • List of Colorado state legislatures
    • Courts of Colorado
      • Colorado Supreme Court
• United States of America
  • United States Congress
    • United States congressional delegations from Colorado
      • List of United States senators from Colorado
      • Colorado's congressional districts
        • List of United States representatives from Colorado