The Nocturne in B major, Op. 40 (B. 47), is a single-movement composition for string orchestra by Antonín Dvořák, published in 1883.

The work originated as the slow movement, Andante religioso, of Dvořák's early string quartet, No. 4 in E minor, of 1870, which was unpublished in his lifetime. The movement was adapted and included in his String Quintet No. 2 in G, of 1875: it was one of two slow movements, and he later withdrew this movement from the quintet.^[1][2][3]

He developed it into this nocturne; the work was published in 1883 by Simrock. It was premiered under the composer's baton at Crystal Palace in London, on 22 March 1885.^[4] As well as the version for string orchestra (B. 47), he made versions for violin and piano (B. 48a) and piano four hands (B. 48b).^[1][2][3]

The work is in the key of B major, and its duration is about 9 minutes. There is a calm atmosphere throughout. After a simple introduction in octaves, there is a long section, featuring a winding melody over a pedal note of F♯ which gives a sense of anticipation; eventually this gives way to a more animated section. Finally there is a return, without the sense of anticipation, to the original texture.^[1][3]