Johann Baptist Strauss II (/straʊs/; German: [ˈjoːhan bapˈtɪst ˈʃtʁaʊs]; 25 October 1825 – 3 June 1899), also known as Johann Strauss Jr., the Younger or the Son (German: Johann Strauß Sohn), was an Austrian composer of light music, particularly dance music and operettas, as well as a violinist. He composed over 500 waltzes, polkas, quadrilles, and other types of dance music, as well as several operettas and a ballet. In his lifetime, he was known as "The Waltz King", and was largely responsible for the popularity of the waltz in the 19th century. Some of Johann Strauss's most famous works include "The Blue Danube", "Kaiser-Walzer" (Emperor Waltz), "Tales from the Vienna Woods", "Frühlingsstimmen", and the "Tritsch-Tratsch-Polka". Among his operettas, Die Fledermaus and Der Zigeunerbaron are the best known.

Strauss was the son of Johann Strauss I and his first wife Maria Anna Streim. Two younger brothers, Josef and Eduard Strauss, also became composers of light music, although they were never as well known as their brother.

Strauss was born into a Catholic family in St Ulrich near Vienna (now a part of Neubau), Austria, on 25 October 1825, to the composer Johann Strauss I and his first wife, Maria Anna Streim. His paternal great-grandfather was a Hungarian Jew – a fact which the Nazis, who lionised Strauss's music as "so German", later tried to conceal. His father did not want him to become a musician but rather a banker. Nevertheless, Strauss Jr. studied the violin secretly as a child with the first violinist of his father's orchestra, Franz Amon. When his father discovered his son secretly practising on a violin one day, he gave him a severe whipping, saying that he was going to beat the music out of the boy. It seems that rather than trying to avoid a Strauss rivalry, the elder Strauss only wanted his son to escape the rigours of a musician's life. It was only when the father abandoned his family for a mistress, Emilie Trampusch [de], that the son was able to concentrate fully on a career as a composer with the support of his mother.

Strauss studied counterpoint and harmony with theorist Professor Joachim Hoffmann, who owned a private music school. His talents were also recognized by composer Joseph Drechsler, who taught him exercises in harmony. It was during that time that he composed his only sacred work, the gradual Tu qui regis totum orbem (1844). His other violin teacher, Anton Kollmann, who was the ballet répétiteur of the Vienna Court Opera, also wrote excellent testimonials for him. Armed with these, he approached the Viennese authorities to apply for a license to perform. He initially formed his small orchestra where he recruited his members at the Zur Stadt Belgrad tavern, where musicians seeking work could be hired easily.

Johann Strauss I's influence over the local entertainment establishments meant that many of them were wary of offering the younger Strauss a contract for fear of angering the father. Strauss Jr. was able to persuade Dommayer's Casino in Hietzing, a suburb of Vienna, to allow him to perform. The elder Strauss, in anger at his son's disobedience and at that of the proprietor, refused to ever play again at Dommayer's Casino, which had been the site of many of his earlier triumphs.

Strauss made his debut at Dommayer's in October 1844, where he performed some of his first works, such as the waltzes "Sinngedichte", Op. 1 and "Gunstwerber", Op. 4 and the polka "Herzenslust", Op. 3. Critics and the press were unanimous in their praise of Strauss's music. A critic for Der Wanderer commented that "Strauss's name will be worthily continued in his son; children and children's children can look forward to the future, and three-quarter time will find a strong footing in him."

Despite the initial fanfare, Strauss found his early years as a composer difficult, but he soon won over audiences after accepting commissions to perform away from home. The first major appointment for the young composer was his award of the honorary position of "Kapellmeister of the 2nd Vienna Citizen's Regiment", which had been left vacant following Joseph Lanner's death two years before.

Vienna was wracked by the revolutions of 1848 in the Austrian Empire, and the intense rivalry between father and son became much more apparent. The son decided to side with the revolutionaries. It was a decision that was professionally disadvantageous, as the Austrian royalty twice denied him the much coveted position of KK Hofballmusikdirektor, which was first designated especially for Johann I in recognition of his musical contributions. Further, the younger Strauss was also arrested by the Viennese authorities for publicly playing "La Marseillaise", but was later acquitted. The elder Strauss remained loyal to the monarchy and composed his "Radetzky March", Op. 228 (dedicated to the Habsburg field marshal Joseph Radetzky von Radetz), which would become one of his best-known compositions.