| Birth Date: | 31 January 1952 |
| Birth Place: | Heemstede, Netherlands |
| Height: | 1.65m (05.41feet) |
| Weight: | 60kg (130lb) |
| Sport: | Swimming |
| Club: | Heemsteedse Zwem- en Poloclub (HPC) |
Maria Alida Catharina "Marjan" Janus (born 31 January 1952) is a former breaststroke swimmer from the Netherlands. In 1968, she won the 100 m breaststroke event at the national championships, beating the long-time favorite Klenie Bimolt. She was therefore selected for the 1968 Summer Olympics, but failed to reach the final.