U.S. Route 6 in Connecticut is the portion of the cross-country U.S. Route 6 within the state of Connecticut. West of Hartford, the route either closely parallels or runs along Interstate 84. I-84 has largely supplanted Route 6 as a through route in western Connecticut. East of Hartford, Route 6 serves as a primary route for travel between Hartford and Providence. The Connecticut Route 6 segments is 116.33 miles (187.21 km) long.

Route 6 enters Connecticut at the New York state line east of the village of Brewster, entering the city of Danbury overlapped with U.S. Route 202. US 6/202 runs for 3.8 miles (6.1 km) in Danbury as a minor arterial road then overlaps with Interstate 84/Route 7 (at Exit 4). The 4-way concurrency of I-84/US 7/US 6/US 202 continues for 3.3 miles (5.3 km), after which Routes 7 and 202 split off from I-84. Route 6 follows I-84 for another 0.8 miles (1.3 km) before returning to surface roads (at Exit 8). The route then goes through the towns of Bethel and Newtown and then overlaps with I-84 again for 6.4 miles (10.3 km) between Newtown and Southbury (from Exits 10 to 15).

After exiting I-84 in Southbury, Route 6 is a surface road again as it passes through the northern Waterbury area suburbs of Southbury, Woodbury, Watertown and Thomaston. US 6 has a 1.0-mile (1.6 km) overlap with the Route 8 expressway in Thomaston.

Route 6 continues as a surface road through the towns of Plymouth, Bristol and Farmington. In Farmington, Route 6 again joins I-84 as it passes through West Hartford, Hartford, East Hartford and Manchester (13.4-mile overlap from Exits 38 to 60). U.S. Route 44 briefly joins I-84/US 6 (for 0.2 miles) as they cross the Connecticut River on the Bulkeley Bridge. After exiting I-84 in Manchester, Route 6 is joined again by Route 44 for 6.9 miles (11.1 km) up to Bolton near the eastern terminus of I-384.

In Bolton, Routes 6 and 44 split. Route 44 follows a more northerly route while Route 6 continues through Bolton, Coventry, Andover and Columbia.

The U.S. Route 6 Willimantic Bypass begins in Columbia, at a four-way at-grade intersection with Route 66. The expressway starts out heading northeast and immediately crosses into Coventry. After crossing the town line, the eastbound and westbound sides of the Route 6 split, with a hill in between them. At the split, the eastbound side of the expressway curves and heads east. At this point, the Hop River State Park Trail passes under both sides of the Route 6. Soon after, the westbound lane also curves, and the two sides of the expressway soon become parallel again. Route 6 passes over Flanders River Road about 0.25 miles (0.40 km) farther. The expressway then crosses the Willimantic River and enters the town of Windham. Right after entering Windham, the New England Central Railroad passes under Route 6. Immediately after this, there is an interchange with Route 32. After the interchange, the expressway enters Mansfield. Mansfield Avenue passes over Route 6, and then there is another interchange. On the eastbound side of the expressway, there is an exit to Mansfield City Road and, about 1 mile (1.6 km) farther, an entrance ramp from Route 195. On the westbound side, there is an exit to North Frontage Road and, about farther, an entrance ramp from Mansfield City Road. Soon after the eastbound entrance ramp joins Route 6, the expressway crosses the Natchaug River and once again enters Windham. 0.5 miles (0.80 km) after entering Windham, the U.S. Route 6 Willimantic Bypass ends at an interchange with Route 66 and Route 6.

Route 6 then continues as a surface road to the towns of Chaplin, Hampton, Brooklyn and Killingly. The unsigned portion of the Connecticut Turnpike (SR 695) then meets with Route 6 just at the Rhode Island state line. Route 6 is a two-lane freeway in the vicinity of its junction with Interstate 395 in Killingly.

Before the creation of the U.S. Highway system in 1926, most of the proposed routing in Connecticut was part of New England Interstate Route 3 (NE-3). There were two places where NE-3 and US 6 were not overlapped. NE-3 began in Bedford, New York at NY 22, entering Connecticut via modern Route 35. NE-3 continued north to Danbury via the old non-expressway alignment of U.S. Route 7. US 6, on the other hand, went east from Brewster on its current alignment, meeting with NE-3 in downtown Danbury.

Another difference in routing is between Manchester and Windham. US 6 originally used a more northern alignment via Coventry, running along present U.S. Route 44 then modern Route 31. NE-3 used current US 6 for its routing. East of Windham, the routes overlapped into Rhode Island. Between 1926 and 1932, NE-3 and US 6 were cosigned where they overlapped. NE-3 was finally deleted in 1932.

I-84 previously was to be an expressway that would connect the modern Interstate 384 with the modern U.S. Route 6 Willimantic Bypass. From here, I-84 would continue to Providence. This idea was planned in the 1960s, but abandoned in 2005.

The Connecticut Department of Transportation had planned since the 1960s to upgrade the segment between Bolton and Columbia to an expressway, connecting I-384 to the existing expressway segment in Windham.[citation needed]. However, this particular segment of Route 6 passes through an environmentally sensitive area centered around the Hop River. Construction was originally planned to begin in the late 1980s, but federal, state, and local officials could not reach an agreement on a feasible route that avoided the Hop River wetlands and development within the towns of Andover, Bolton, Coventry, and Columbia. The affected towns, CONNDOT and the Connecticut Department of Environmental Protection favored a northern alignment (Alternative 133B), which would avoid the town centers and nearby wetlands. The Environmental Protection Agency, Army Corps of Engineers, and Federal Highway Administration favored a southerly alignment (Alternative 133 18/25) that would cut through residential and commercial areas as well as the Hop River's adjoining wetlands.

Despite opposition from CONNDOT, the Connecticut DEP, and affected towns, the FHWA issued a Record of Decision and the Army Corps of Engineers issued required permits for Alternative 133 18/25 (southern alignment) in 2001. State and local officials continued to press the Corps of Engineers to approve the northern alignment. Due to the impasse between state, local, and federal officials, federal funds for the bypass were withdrawn in 2003. In 2005, the Capitol Region Council of Governments and CONNDOT removed the Route 6 bypass from planning, hence CONNDOT effectively abandoned further study of the bypass in lieu of upgrading the existing road.

As of 2007, CONNDOT is making safety improvements and capacity upgrades to the existing US-6 through Andover, Bolton, and Columbia.

There have been several routes signed as US 6A in the state. No bannered routes currently exist.