There are a number of claims to this title, but some are questionable as regards to being a true multicellular organism. Prior findings may be only groupings of similar, single-cell organisms, and do not necessarily represent independent, multicellular life forms.
However, the latest report of a one billion year old fossil which contains two distinct cell types within the same "structure" may prove to be the first true multicellular form. Quoting from the link below*:
"We have found a primitive spherical organism made up of an arrangement of two distinct cell types, the first step towards a complex multicellular structure, something which has never been described before in the fossil record."
Doubtless these findings will stir up some debate as to their nature, and for any older fossils where such claims have also been made.
* https://phys.org/news/2021-04-billio...evolution.html
For more on this:
https://www.bbc.co.uk/news/uk-scotla...lands-56917272