Suffolk County Transit (SCT) is the public transit bus system of Suffolk County, New York, on Long Island, and is an agency of the Suffolk County government. It was founded in 1980 as a county-run oversight and funding agency for a group of private contract operators which previously provided such services on their own. While the physical maintenance and operation of the buses continue to be provided by these contractors, other matters ranging from bus purchases to route and schedule planning to fare rules are set by Suffolk Transit itself.

Though serving the entirety of Suffolk County, the one exception is in Huntington – located in the northwestern part of the county, where that town's private operator declined to join SCT. Instead, Huntington took over that town's system which became Huntington Area Rapid Transit, or HART. Most of HART's routes do connect to both Suffolk County Transit and Nassau Inter-County Express, transfers can be made between HART and SCT. In addition, the village of Patchogue previously had its own local bus service, but discontinued it.

Prior to the formation of Suffolk County Transit in 1980, public bus services within Suffolk County were provided by private operators. These included the Bornscheuer Bus Company, which served Amityville, Copiague, Lindenhurst, West Babylon, and Babylon. An affiliated company – EBT, Inc. (an affiliate of school bus company Educational Bus Transportation, Inc.) – continues to operate several of these routes under contract with Suffolk County Transit.

In Central Suffolk, the East Patchogue-based Louis A. Fuoco Bus Line (1962–1992) served Patchogue, East Patchogue, Hagerman, Bellport, South Haven, Mastic, Mastic Beach, Port Jefferson, Medford, Coram, Ridge, Calverton, and Riverhead; it continued to exist during early years of Suffolk County Transit. Additionally, although Coram Bus Service (1958–1992) primarily operated school buses, that company began operating mass transit routes in the Town of Brookhaven towards locations as far west as Commack and East Northport, and as far east as Riverhead.

In Western Suffolk and on the North Shore, the Huntington Coach Corporation (1927–present) served Huntington, East Farmingdale, Melville, and Halesite. Quinn's Bus Lines (and later Coram Bus Service, after the two merged in 1969) mainly served the area between Port Jefferson and Wading River – especially after the demise of the Wading River Extension of the Port Jefferson Branch.

In the western parts of the Town of Islip, Suffolk Bus Corporation (1946–present) served Bay Shore, Brentwood, West Brentwood, Central Islip, Islip, and Babylon; it continues to operate several of these routes under contract with Suffolk County Transit.

On the East End, Sunrise Coach Lines (1946–present) served Greenport, Riverhead, Southampton, Sag Harbor, and East Hampton; it continues to operate several of these routes under contract with joint venture Twin Forks Transit for the routes they originally operated for Suffolk County Transit. In 2006, Sunrise Coach's EXPRESS Routes were purchased by Hampton Jitney. This joint venture was discontinued on October 29, 2023, as the discontinuation of the S92 – which had been co-operated by Sunrise and Hampton Jitney – rendered it obsolete.

Utility Lines – a Bee Line, Inc. subsidiary – ran from Patchogue via Montauk Highway, thence Merrick Road into Nassau County. This line was merged into the Metropolitan Suburban Bus Authority (MSBA) in 1973, but funding and service disputes lead to the splitting of that route between the N19 west of Babylon and S40 east of Babylon.