Summary: For a nontrivial connected graph \(G\), let \(c\: V(G)\to \mathbb {N}\) be a vertex colouring of \(G\), where adjacent vertices may be coloured the same. For a vertex \(v\) of \(G\), the neighbourhood colour set \(\operatorname {NC} (v)\) is the set of colours of the neighbours of \(v\). The colouring \(c\) is called a set colouring if \(\operatorname {NC} (u)\neq \operatorname {NC} (v)\) for every pair \(u,v\) of adjacent vertices of \(G\). The minimum number of colours required of such a colouring is called the set chromatic number \(\chi _{s}(G)\). A study is made of the set chromatic number of the join \(G + H\) of two graphs \(G\) and \(H\). Sharp lower and upper bounds are established for \(\chi _{s}(G+H)\) in terms of \(\chi _{s}(G)\), \(\chi _{s}(H)\) and the clique numbers \(\omega (G)\) and \(\omega (H)\).