The Baltimore Steam Packet Company, nicknamed the Old Bay Line, was an American steamship line from 1840 to 1962 that provided overnight steamboat service on Chesapeake Bay, primarily between Baltimore, Maryland, and Norfolk, Virginia. Called a "packet" for the mail packets carried on government mail contracts, the term in the 19th century came to mean a steamer line operating on a regular, fixed daily schedule between two or more cities. When it closed in 1962 after 122 years of existence, it was the last surviving overnight steamship passenger service in the United States.

In addition to regularly calling on Baltimore and Norfolk, the Baltimore Steam Packet Company at various times provided freight, passenger and vehicle transport to Washington, D.C., Old Point Comfort, and Richmond, Virginia. The Old Bay Line, as it came to be known by the 1860s, was acclaimed for its genteel service and fine dining, serving Chesapeake Bay specialties. Walter Lord, famed author of A Night to Remember (and whose grandfather had been the packet line's president from 1893 to 1899), mused that its reputation for excellent service was attributable to "some magical blending of the best in the North and the South, made possible by the Company's unique role in 'bridging' the two sections...the North contributed its tradition of mechanical proficiency, making the ships so reliable; while the South contributed its gracious ease".

In 1947 a former Old Bay Line steamship, President Warfield, became Exodus, carrying Jewish refugees from Europe in an unsuccessful attempt to emigrate to Mandatory Palestine. The voyage was commemorated in a book in 1958 and movie in 1960.

Just seven years after Robert Fulton proved the commercial viability of steam-powered ships with his North River Steamboat (more commonly known today as Clermont) in 1807, small wood-burning steamers began to ply the Chesapeake Bay. Before the arrival of railroads and river steamboats in the early 19th century, overland travel was exceedingly slow and tedious. Rivers were the main means of transportation and most cities were founded on them. This was especially so in North America, where journeys over vast distances of hundreds or even thousands of miles required months of hazardous, uncomfortable travel by stagecoach or wagon on rutted, unpaved trails. In the 1830s, railroads were being built, but the technology was crude and average passenger train speed was only 12 miles per hour (19 km/h). Perhaps more importantly, most early railroads did not connect. It would be many years before the various lines were knitted together to make intercity rail travel in the U.S. a reality. Not until 1863, for example, was it possible to travel between New York City and Washington, D.C., without changing trains en route.

In this period, steamships on rivers such as the Ohio and Mississippi or large inland bodies of water such as the Great Lakes and the Chesapeake Bay offered a comfortable and relatively fast mode of transportation. The first steamboat to serve Baltimore was the locally built Chesapeake, constructed in 1813 to link Baltimore with Philadelphia, Pennsylvania. Operated by the Union Line, the boat connected with a stagecoach for the overland portion of the journey. Two years later, the Briscoe-Partridge Line's Eagle was the first steamboat to sail the length of Chesapeake Bay.

The direct ancestor of the Baltimore Steam Packet Company was the Maryland & Virginia Steam Boat Company formed in 1828 to link Baltimore, Richmond, and Norfolk, traversing the Chesapeake Bay and the James River. By 1839, the Maryland & Virginia was heavily in debt from the purchase of two new, large ships the year before: the 210-foot (64 m) long Alabama and the 173-foot (53 m) Jewess. Alabama was costly to operate and proved impractical for Chesapeake Bay operations, causing the bankruptcy of the Maryland & Virginia later that year.

When the Maryland & Virginia collapsed in late 1839, the Maryland legislature convened to grant a charter to the Baltimore Steam Packet Company, organized in Baltimore to provide overnight steamship service on the Chesapeake Bay. The company's incorporators were Benjamin Bush, Andrew F. Henderson (who became the line's first president), John B. Howell, Thomas Kelso (who would become a director of the line), John S. McKim, Samuel McDonald, Gen. William McDonald, Robert A. Taylor, and Joel Vickers, all of Baltimore.

The company was granted a 20-year charter on March 18, 1840, by the Maryland legislature and then acquired three of the former Maryland & Virginia's steamboats: Pocahontas, Georgia, and Jewess. The company began overnight paddlewheel steamship passenger and freight service daily except Sundays between Baltimore and Norfolk. By 1848, the company's steamship Herald was making the trip in less than 12 hours, a time which the line would maintain until the end in 1962. An affiliate, the Powhatan Line, started service between Norfolk and Richmond in 1845, interchanging freight and passengers with the Old Bay Line.