Interstate 84 is an East–West Interstate highway across the state of Connecticut into Danbury, Waterbury, Hartford and Union.

The interstate's first exit is at the state line, where it enters the city of Danbury. Here it is designated the Yankee Expressway. Two miles to the east, where US 7 comes in from the south near Danbury Fair Mall to join I-84, it turns to the north. At the next exit, routes 6 and 202 join the highway.

The four-way concurrency ends after 3 miles (4.8 km), when 7 and 202 split off north towards New Milford. Route 6 leaves the interstate at the next exit, and I-84 continues east across the countryside. At Exit 11 it turns to the northeast and descends to cross the Housatonic River on the Rochambeau Bridge, into New Haven County, then climbs into higher ground to the city of Waterbury, which it passes on an elevated viaduct with the eastbound and westbound lanes on different levels. Here the CT 8 expressway intersects.

The eastern heading continues past Waterbury to Milldale, where Interstate 691 splits off to the east. This section has many left-hand exits and entrances and sharp curves, which were built for a planned network of freeways. I-84 heads northeast towards New Britain and Hartford, the state capital and the largest community along its eastern length. After intersecting Interstate 91, the road crosses the Connecticut River on the Bulkeley Bridge, oldest on the Interstate system, then becomes the Wilbur Cross Highway and continues towards the northeast.

I-84 climbs steadily from the Connecticut River Valley and reaches the highest Interstate elevation in Connecticut near exit 73 in Union at 1,008 feet (307 m). The last exit in Connecticut is Exit 74, for Route 171 before crossing the Massachusetts border.

Interstate 84 was originally to head east from Hartford, Connecticut to Providence, Rhode Island.

The original route of Interstate 84 would have used present-day Interstate 384 to Bolton, Connecticut, then along a never-built section of freeway that would have connected to the US 6 bypass around Willimantic, Connecticut. Another never-built freeway section would have connected it to Interstate 395 and extended I-84 onto State Road 695 in Connecticut, the easternmost portion of the Connecticut Turnpike in Killingly, Connecticut. From there, it would have roughly followed US 6 through western Rhode Island to connect to the present-day US 6 freeway in Johnston. From there, a freeway from Olneyville Square to the Interstate 95–Interstate 195 interchange was briefly considered, but abandoned in favor of what later became the Route 6-10 Connector.

Sections of I-84 in Connecticut were reconstructed and widened in the 1980s. The former I-86 portion from East Hartford to the Massachusetts state line was completely rebuilt from a narrow 4-lane freeway to a much wider profile ranging from 6 lanes at the Massachusetts state line, expanding to 8 lanes in Vernon, to 12 lanes with HOV lanes in East Hartford. Another section through Danbury was widened from 4 lanes to 6 lanes in 1985 and 1986. Widening of the highway through Danbury was funded by Union Carbide as part of building its world headquarters in Danbury.

Though the route was basically set in stone in Connecticut, many issues remained in Rhode Island, the biggest of which were major environmental concerns about how the freeway would affect the Scituate Reservoir, which is the main drinking water supply for Providence.

In the 1992 long-range transportation plan released by the Rhode Island Department of Transportation, a freeway has been added along the original route of Interstate 84 that will connect to the CT 695 freeway on the Rhode Island–Connecticut border.

The section of I-84 between East Hartford, Connecticut (at the present-day junction with Interstate 384) and Sturbridge, Massachusetts (Interstate 90) was for a time signed as Interstate 86 (unrelated to present-day Interstate 86 in New York and Pennsylvania). Signs stating "I-84 Ends, I-86 to Boston" (eastbound) and "I-86 Ends, I-84 to Hartford" (westbound) were posted where the change took place. Exit numbering on I-86 was that of the road's predecessor, Route 15, in a sequence beginning on New York's Hutchinson River Parkway. Exits were renumbered to correspond with the rest of I-84 in Connecticut when the road was redesignated in 1984. The present I-384 as well as the present US 6 bypass near Willimantic, both of which were a part of what was then I-84's planned easterly continuation, were also numbered I-84 prior to 1984 even though they lacked any direct connection to the rest of I-84 at that time. (One had to use Silver Lane in East Hartford to travel between the two stretches of the highway.) These two sections were re-numbered I-384 and Rte 6 when what was then I-86 was re-numbered I-84.

A widening project along the congested stretch of I-84 through Waterbury and Cheshire, Connecticut has been beset by cost overruns, delays, and construction defects involving storm drains, as state and federal officials have launched criminal investigations stemming from this project. This episode has waned local enthusiasm for a proposed $2 billion reconstruction of the Mixmaster interchange in downtown Waterbury. Cost estimates for the Mixmaster replacement have increased to $3 billion. CT Attorney General Richard Blumenthal has begun a lawsuit against the contractor and an engineering firm in response to threats from the U.S. DOT to withhold funds from the project. On May 18, 2007, the Waterbury Republican-American reported this area had defective light poles, while Governor M. Jodi Rell released an audit report of the construction disaster. A number of DOT personnel were either fired or reprimanded following the scandal. Meanwhile[when?] the FBI and a federal grand jury are investigating the now-defunct construction company and the same DOT officials, which may eventually lead to criminal charges in the case.