List of 64 counties in the U.S. state of Colorado:

1. Adams County formed in 1902 from part of Arapahoe county.
2. Alamosa County formed in 1913 from parts of Conejos and Costilla counties.
3. Arapahoe County one of the original 16 counties.
4. Archuleta County formed in 1885 from part of Conejos county.
5. Baca County formed in 1889 from part of Las Animas county.
6. Bent County formed in 1874 from part of Greenwood county.
7. Boulder County one of the original 16 counties.
8. Broomfield County formed in 2001 from parts of Adams, Boulder, Jefferson and Weld counties.
9. Chaffee County formed in 1879 from part of Lake county.
10. Cheyenne County formed in 1889 from parts of Bent and Elbert counties.
11. Clear Creek County one of the original 16 counties.
12. Conejos County one of the original 16 counties. Originally also known as Guadalupe county
13. Costilla County one of the original 16 counties.
14. Crowley County formed in 1911 from part of Otero county.
15. Custer County formed in 1877 from part of Fremont county.
16. Delta County formed in 1883 from part of Gunnison county.
17. Denver County formed in 1902 from part of Arapahoe county.
18. Dolores County formed in 1881 from part of San Juan county.
19. Douglas County one of the original 16 counties.
20. Eagle County formed in 1883 from part of Summit county.
21. El Paso County one of the original 16 counties.
22. Elbert County formed in 1874 from parts of Douglas and Greenwood counties.
23. Fremont County one of the original 16 counties.
24. Garfield County formed in 1883 from part of Summit county.
25. Gilpin County one of the original 16 counties.
26. Grand County formed in 1874 from part of Summit county.
27. Gunnison County formed in 1877 from part of Lake county.
28. Hinsdale County formed in 1874 from parts of Conejos and Lake counties.
29. Huerfano County one of the original 16 counties.
30. Jackson County formed in 1909 from part of Larimer county.
31. Jefferson County one of the original 16 counties.
32. Kiowa County formed in 1889 from part of Bent county.
33. Kit Carson County formed in 1889 from part of Elbert county.
34. La Plata County formed in 1874 from parts of Conejos and Lake counties.
35. Lake County one of the original 16 counties.
36. Larimer County one of the original 16 counties.
37. Las Animas County formed in 1866 from part of Huerfano county.
38. Lincoln County formed in 1889 from parts of Bent and Elbert counties.
39. Logan County formed in 1887 from part of Weld county.
40. Mesa County formed in 1883 from part of Gunnison county.
41. Mineral County formed in 1893 from parts of Hindsdale and Rio Grande counties.
42. Moffat County formed in 1911 from part of Routt county.
43. Montezuma County formed in 1889 from part of La Plata county.
44. Montrose County formed in 1883 from part of Gunnison county.
45. Morgan County formed in 1889 from part of Weld county.
46. Otero County formed in 1889 from part of Bent county.
47. Ouray County formed in 1877 from parts of Lake and San Juan counties.
48. Park County one of the original 16 counties.
49. Phillips County formed in 1889 from part of Logan county.
50. Pitkin County formed in 1881 from part of Gunnison county.
51. Prowers County formed in 1889 from part of Bent county.
52. Pueblo County one of the original 16 counties.
53. Rio Blanco County formed in 1889 from part of Garfield county.
54. Rio Grande County formed in 1874 from parts of Conejos and Costilla counties.
55. Routt County formed in 1877 from part of Grand county.
56. Saguache County formed in 1866 from parts of Costilla and Lake counties.
57. San Juan County formed in 1876 from part of Lake county.
58. San Miguel County formed in 1883 from part of Ouray county.
59. Sedgwick County formed in 1889 from part of Logan county.
60. Summit County one of the original 16 counties.
61. Teller County formed in 1899 from part of El Paso county.
62. Washington County formed in 1887 from part of Weld county.
63. Weld County one of the original 16 counties.
64. Yuma County formed in 1889 from part of Washington county.

Defunct counties

1. Greenwood County formed in 1870 from part of Huerfano county and the Arapahoe and Cheyenne Reserve. In 1874 was abolished and Bent and Elbert counties created.
2. Platte County formed in 1872 from part of Weld county. In 1874 was abolished and its territory returned to Weld county.