Route 51 is a bus route operated by the Maryland Transit Administration in Baltimore and its suburbs. The line currently runs from the center of Towson to the Inner Harbor in Downtown Baltimore, serving the Charles Street corridor. Route 51 replaced Route 11 on June 18, 2017 due to the BaltimoreLink bus system overhaul.

As part of the Bus Network Improvement Project, the 11 was truncated to run between Towson and the Inner Harbor, with the eastern portion of the route served by the new Route 31, effective 22 February 2015.

Previously, Route 11 started operating between Towson and Canton on February 17, 2008, following modification from its previous route that had been in place for several decades. The line had always served the Charles Street corridor throughout its existence, but its other parts had varied. Numerous branches off of Charles Street and the route operating southwest from downtown Baltimore to Riverview via Washington Boulevard have existed over the years.

The changes that went into effect in 2008 had originally been proposed nearly a decade earlier, but had especially been pushed since 2005. These mostly involved replacing the southern portion of the route with an extension on Route 36.

The routing structure along the Charles Street corridor is the successor to the 11 Bedford Square streetcar and bus Route O. In February 2015 as part of a Bus Network Improvement Project the southeast would be discounted terminating at Light & Lee streets and all Southeast service be transferred to the new 31.

Initially, a railroad along Charles Street, known as the "Charles Street Line," began operation in 1862.

The no. 11 streetcar along Charles Street began operation in 1929. The no. 11 designation had previously been used for two jerkwater lines along Roland Avenue and then in Dundalk.

In 1953, the line was extended to Murray Hill when it was combined with Route 56, which had operated since 1948 north of Bedford Square.