In computer science, a substring index is a data structure which gives substring search in a text or text collection in sublinear time. If you have a document ${\displaystyle S}$ of length ${\displaystyle n}$, or a set of documents ${\displaystyle D=\{S^{1},S^{2},\dots ,S^{d}\}}$ of total length ${\displaystyle n}$, you can locate all occurrences of a pattern ${\displaystyle P}$ in ${\displaystyle o(n)}$ time. (See Big O notation.)

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The phrase full-text index is also often used for an index of all substrings of a text. But this is ambiguous, as it is also used for regular word indexes such as inverted files and document retrieval. See full text search.

Substring indexes include:

• Suffix tree
• Suffix array
• N-gram index, an inverted file for all N-grams of the text
• Compressed suffix array^[1]
• FM-index
• LZ-index