Join may refer to:
- Join (law), to include additional counts or additional defendants on an indictment
- In mathematics:
- Join (mathematics), a least upper bound of sets orders in lattice theory
- Join (topology), an operation combining two topological spaces
- Join (category theory), an operation combining two categories
- Join (simplicial sets), an operation combining two simplicial sets
- Join (sigma algebra), a refinement of sigma algebras
- Join (algebraic geometry), a union of lines between two varieties
- In computing:
- Join (relational algebra), a binary operation on tuples corresponding to the relation join of SQL
- Join (SQL), relational join, a binary operation on SQL and relational database tables
- join (Unix), a Unix command similar to relational join
- Join-calculus, a process calculus developed at INRIA for the design of distributed programming languages
- Join-pattern, generalization of Join-calculus
- Joins (concurrency library), a concurrent computing API from Microsoft Research
- Join (relational algebra), a binary operation on tuples corresponding to the relation join of SQL
- Join Network Studio of NENU, a non-profit organization of Northeast Normal University
- Joins.com, the website for South Korean newspaper JoongAng Ilbo
- Joining (woodworking), woodworking processes of combining two or more pieces of wood together, generally through the use of nails.