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Undeciphered writing systems are any written or inscribed symbols for which no decipherment has achieved broad acceptance in the specialist literature.
While most examples are ancient, a small number are medieval or modern. In some cases the evidence is too limited to determine whether the marks represent a true writing system, a form of proto-writing, or a set of non-linguistic symbols; modern artistic traditions such as asemic writing likewise imitate the appearance of writing while intentionally withholding stable linguistic meaning. It is not clear all these cases were intended as writing systems with linguistic function or representation, and some inscribed symbols or characters may have other explanations such as being artwork.
Difficulties in decipherment commonly arise from one or more of the following: the absence of bilingual texts or other external "anchors" (such as securely identifiable names or dates); uncertain or unknown underlying languages (including possible language isolates); small corpora; and damage or loss of archaeological context needed to test proposed readings against use and genre. In some corpora, an additional difficulty is uncertainty over whether the signs constitute writing at all (as has often been argued for the Vinča symbols).
Various decipherment claims have been proposed for several items listed here (including the Indus script, the Phaistos Disc, and the Isthmian/Epi-Olmec script), but these proposals remain disputed and have not achieved broad acceptance in the scholarly literature unless otherwise noted.
This list includes writing systems and sign systems that are commonly described in reliable secondary sources as undeciphered, not fully deciphered, or of uncertain decipherment. Items of disputed authenticity or disputed status as writing are explicitly identified as such.
Certain forms of proto-writing remain undeciphered and, because of limited evidence and uncertain linguistic affiliation, may never be deciphered.

Many Mesoamerican glyphic traditions remain only partly understood, with the Classic/Lowland Maya script being the best-attested and most extensively deciphered. Other regional traditions (including those often grouped under "Mixteca-Puebla" or pictorial-historical codex traditions ultimately descended from the Teotihuacan tradition, most notably represented by Aztec hieroglyphs and Mixtec hieroglyphs) have been interpreted in the literature to varying extents, but are not universally recognised as a writing system, with some researchers considering it a mostly non-linguistic form of expression. Yet, in recent years[as of?] new research has appeared confirming the existence of logosyllabic elements ("glyphs") intermixed with pictorial images. Nevertheless, new approaches have emerged, integrating the pictorial images as key in the linguistic interpretation of all elements in the codices, alongside the logograms and phonograms. These pictorial images would represent greater units of meaning, namely semantograms, also called embedded meanings, than a lexeme (logograms) or a sound (phonogram) which would become of paramount importance to achieve a coherent linguistic message in the codices. Thus, Mixteca-Puebla writing would constitute a form of non-linear writing.
False writing systems are glyphs that appear to be writing but are not. False writing cannot be deciphered because it has no stable semantic meaning. This includes many forms of asemic writing created for artistic purposes. A prominent example is the Codex Seraphinianus.
Another related concept is that of undeciphered cryptograms or cipher messages. These are not writing systems per se, but enciphered forms of texts written in known systems. Some have become famous and are listed in list of ciphertexts.
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