A person who has learned techniques that enable extraordinary feats of memory.

an expert in the use of mnemonics; someone able to perform unusual feats of memory

The title Mnemonist (derived from the term Mnemonic) refers to an individual with the ability to remember and recall unusually long lists of data, for example: unfamiliar names, lists of numbers, entries in books, etc. Such individuals have also been described as possessing an Eidetic memory, although whether such abilities are innate behaviour or somehow learned appears somewhat contentious. Nonetheless, individuals famed as Mnemonists have become part of popular myth, lore, fiction, contemporary media, and, indeed, subjects of scientific enquiry.