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 (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 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 or screws
• Joining (metalworking), metalworking processes which combine two or more pieces of metal together, typically by the use of screws or welding