Yes. When the tiny jellyfish Turritopsis dohrnii is hurt, starving, or stressed, it can sink to the seafloor, dissolve its adult body back into its baby stage, and start its life completely over.
Most animals get one pass through life: baby, adult, old age, done. This jellyfish carries a restart. When life goes wrong — an injury, no food, harsh water — the adult stops swimming, sinks, and collapses into a shapeless blob. From that blob grows a polyp, the stalk-like baby stage this species normally passes through once, early in life. The polyp then buds off fresh young jellyfish, genetically identical to the adult that melted down.
It's less like healing and more like a butterfly crawling back into its chrysalis and coming out a caterpillar.
Ordinary adult cells are locked into their jobs — a muscle cell stays a muscle cell. This jellyfish's cells can unlock. In a process called transdifferentiation, adult cells convert directly into other cell types needed to rebuild the polyp stage. That ability to rewind development is why scientists study this fingernail-sized animal so closely: it is doing, naturally, something medicine can barely do at all.
Careful wording matters here. The jellyfish is called "biologically immortal" because, in principle, there's no known limit to how many times it can reset — no built-in old-age countdown. In the real ocean, though, most get eaten or catch disease long before any reset matters. It can cheat aging. It cannot cheat a hungry turtle.
See the short version in the LOE PRESSURE "Things You Didn't Know" series — "The Jellyfish That Becomes a Baby Again."
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Sources: ScienceAlert, "The 'Immortal Jellyfish' Can Age in Reverse" — https://www.sciencealert.com/the-immortal-jellyfish-can-age-in-reverse-and-possibly-live-forever; Wikipedia, Turritopsis dohrnii — https://en.wikipedia.org/wiki/Turritopsis_dohrnii.