In the U.S. state of Colorado, Interstate 25 follows the north–south corridor through Colorado Springs and Denver. The route is concurrent with U.S. Highway 87 through the entire length of the state. I-25 replaced U.S. Highway 87 and most of U.S. Highway 85 for through traffic.

Historical nicknames for this route have included the Valley Highway (through Denver), Monument Valley Highway (through Colorado Springs), and the Pueblo Freeway (through Pueblo).

As I-25 leaves Denver, the route continues up north and interchanges with I-76, I-270, and the Denver-Boulder Turnpike (US 36). Due to the complexity of this triangle-shaped interchange, it was known to be one of many malfunction junctions throughout the United States. Beyond that interchange and exit 220, I-25 slips its way through a narrow path between the Badding Reservoir (west side) and the Croke Lake (east side).

At exit 228, I-25 interchanges with the northern termini of E-470 and Northwest Parkway at a stack interchange, with the Larkridge Mall just to the north, served by 160th Avenue (SH 7). As I-25 continues north, it passes through a medley of lakes and reservoirs on the way north to Wyoming and generally stays rural in nature to the east of the Front Range. The interstate enters the Fort Collins/Loveland metro area at exit 255, serving Loveland and Greeley to the east at exits 255 and 257, and continuing north to the Fort Collins city limits south of Harmony Road. The highway runs on the east side of Fort Collins, serving Colorado State University at exits 268 and 269. After exit 271, I-25 leaves Fort Collins and continues through rural grasslands to the Wyoming border. Afterwards, the interstate gradually makes a north-northeast turn as it heads for the Wyoming state line.

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