The B45 is a bus route that constitutes a public transit line operating in Brooklyn, New York City, mainly along Atlantic Avenue, Washington Avenue, Sterling Place, and St. Johns Place between Downtown Brooklyn and Crown Heights. It is operated by the MTA New York City Transit Authority. Its precursor was a streetcar line that began operation in 1877, and was known as the St. Johns Place Line. The route became a bus line in 1947.

Atlantic Avenue is a major thoroughfare in the New York City boroughs of Brooklyn and Queens. It stretches from the Brooklyn waterfront on the East River all the way to Jamaica, Queens. Atlantic Avenue runs parallel to Fulton Street for much of its course through Brooklyn, where it serves as a border between the neighborhoods of Prospect Heights and Fort Greene and between Bedford-Stuyvesant and Crown Heights, and between Brooklyn Heights and Cobble Hill. This stretch of avenue is known for having a high rate of pedestrian fatalities and has been described as "the killing fields of the city."