Ero s onoga svijeta (usually translated as Ero the Joker, literally Ero from that world) is a comic opera in three acts by Jakov Gotovac, with a libretto by Milan Begović based on a folk tale. The genesis of the opera was at Vrlička Česma in the town of Vrlika, a hometown of Milan Begović.

According to Croatian musicologist Josip Andreis, Ero s onoga svijeta is "not only the most successful Croatian comic opera to this day, but also the only Croatian opera with a presence in the theaters abroad".

Composition of the opera began on October 10, 1932 and progressed in three stages, being finished on May 8, 1935. The first performance was on November 2, 1935 at the Croatian National Theatre (Hrvatsko narodno kazalište) in Zagreb, and the opera has since become the most performed work of South Slavic music literature.

The first performance was conducted by Gotovac himself, and he felt that the opera was nicely received by the audience. In Novosti Milan Katić described the opera in superlatives, and in Belgrade Pravda Stražičić shared this positive sentiment. An often-quoted dismissive opinion ("And again a Croatian composer wrote an opera in vain") allegedly found in Lujo Šafranek-Kavić's review in Jutarnji list, a Zagreb-based newspaper, has been found by recent research to be a fabrication. The actual review was generally very positive, with Šafranek-Kavić giving particular praise to the Gotovac score, while having reservations about the quality of the libretto.

Ero the Joker saw its first performance outside Yugoslavia in Brno, Czechoslovakia, translated into Czech in 1936, and after that it came back to the National Theater (Narodno pozorište) in Belgrade, Yugoslavia on April 17, 1937. It was next put on stage more than ten years later, on February 27, 1948, in the Serbian National Theatre (Srpsko narodno pozorište) in Novi Sad where it has been put on five times since. All totaled, Ero the Joker has found its way to the stages of more than 80 world theaters, and has been translated into 9 languages. Between 1935 and 2010 it has seen 660 performances in the Croatian National Theatre in Zagreb alone.

Gotovac and Begović found the basics for the opera's music and text in the folklore of many South Slavic groups, ranging from Dalmatian folklore (Opera Finale) to songs from Kosovo (opening chorus Duni mi, duni, lađane).