The Symphony No. 1 in D major by Gustav Mahler was mainly composed between late 1887 and March 1888, though it incorporates music Mahler had composed for previous works. It was composed while Mahler was second conductor at the Leipzig Opera in Germany. Although in his letters Mahler almost always referred to the work as a symphony, the first two performances described it as a symphonic poem and as a tone poem in symphonic form, respectively. The work was premièred at the Vigadó Concert Hall in Budapest, Hungary, in 1889, but was not well-received. Mahler made some major revisions for the second performance, given at Hamburg, Germany, in October 1893; further alterations were made in the years prior to the first publication, in late 1898. Some modern performances and recordings give the work the title Titan, despite the fact that Mahler only used this label for the second and third performances, and never after the work had reached its definitive four-movement form in 1896.

Gustav Mahler was an Austro-Bohemian Romantic composer, and one of the leading conductors of his generation. As a composer he acted as a bridge between the 19th-century Austro-German tradition and the modernism of the early 20th century. While in his lifetime his status as a conductor was established beyond question, his own music gained wide popularity only after periods of relative neglect, which included a ban on its performance in much of Europe during the Nazi era. After 1945 his compositions were rediscovered by a new generation of listeners; Mahler then became one of the most frequently performed and recorded of all composers, a position he has sustained into the 21st century.