| Official Name: | Município de Casimiro de Abreu |
| Mapsize: | 250px |
| Pushpin Map: | Brazil |
| Pushpin Map Caption: | Location of Casimiro de Abreu in Brazil |
| Subdivision Type: | Country |
| Subdivision Type1: | Region |
| Subdivision Type2: | State |
| Subdivision Name: |  Brazil |
| Subdivision Name1: | Southeast |
| Subdivision Name2: | Rio de Janeiro |
| Leader Title: | Prefeito |
| Leader Name: | Paulo Cezar Dames Passos (PSB) |
| Area Total Km2: | 462.918 |
| Population As Of: | 2022 [1] |
| Population Total: | 46,110 |
| Population Density Km2: | auto |
| Timezone: | BRT |
| Utc Offset: | -3 |
| Coordinates: | -22.4808°N -42.2039°W |
| Elevation M: | 17 |
| Website: | http://www.casimirodeabreu.rj.gov.br |
| Settlement Type: | Municipality |
| Casimiro de Abreu | |
Casimiro de Abreu (pronounced as /pt/) (formerly known as Barra de São João pronounced as /pt/) is a municipality located in the Brazilian state of Rio de Janeiro. Its population was 46,110 (2022) and its area is 462 km2.
The city was called Indaiaçu until it was rebaptized in 1925 to homage the poet Casimiro de Abreu, who was born in their coastal district Barra de São João.
The municipality contains part of the Central Rio de Janeiro Atlantic Forest Mosaic, created in 2006.It holds part of the União Biological Reserve, home to a population of endangered golden lion tamarin.It contains of the Macaé de Cima Environmental Protection Area, created in 2001.