| Country: | Spain |
| Internationalist Socialist Workers' Party | |
| Native Name: | Partido Obrero Socialista Internacionalista |
| Foundation: | 1980 |
| Ideology: | Trotskyism Marxism |
| Position: | Far-left |
| Website: | www.posicuarta.org |
The Internationalist Socialist Workers' Party (Spanish; Castilian: Partido Obrero Socialista Internacionalista, POSI) is a Spanish political party of trotskyist ideology founded in 1980.[1] It was a member of the Fourth International (ICR).