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In computer science, a multimap (sometimes also multihash, multidict or multidictionary) is a generalization of a map or associative array abstract data type in which more than one value may be associated with and returned for a given key. Both map and multimap are particular cases of containers (for example, see C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.

• In a student enrollment system, where students may be enrolled in multiple classes simultaneously, there might be an association for each enrollment of a student in a course, where the key is the student ID and the value is the course ID. If a student is enrolled in three courses, there will be three associations containing the same key.
• The index of a book may report any number of references for a given index term, and thus may be coded as a multimap from index terms to any number of reference locations or pages.
• Querystrings may have multiple values associated with a single field. This is commonly generated when a web form allows multiple check boxes or selections to be chosen in response to a single form element.

C++'s Standard Template Library provides the multimap container for the sorted multimap using a self-balancing binary search tree,^[1] and SGI's STL extension provides the hash_multimap container, which implements a multimap using a hash table.^[2]

As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap.^[3]

Quiver provides a Multimap for Dart.^[4]

Apache Commons Collections provides a MultiMap interface for Java.^[5] It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.^[6]

Google Guava provides a Multimap interface and implementations of it.^[7]

Kotlin does not have explicit support for multimaps,^[8] but can implement them using Maps with containers^[9] for the value type. E.g. a Map<User, List<Book>> can associate each User with a list of Books.

Python provides a collections.defaultdict class that can be used to create a multimap. The user can instantiate the class as collections.defaultdict(list).

OCaml's standard library module Hashtbl implements a hash table where it's possible to store multiple values for a key.

The Scala programming language's API also provides Multimap and implementations.^[10]

• Multiset for the case where same item can appear several times