Who discovered the nucleus and in which year? How it was discovered?

Nucleus in the cell was discovered by Antonie van Leeuwenhoek in the 18th century. It was the first organelle to be discovered.

When Antonie van Leeuwenhoek was observing the red blood cells of salmon, he found a 'lumen', the nucleus, in the red blood cells of salmon, and came to the point that, unlike mammalian red blood cells, those of other vertebrates still contain nuclei.

It was also described by Franz Bauer in 1804 and by Scottish botanist Robert Brown in more detail in 1831.