A graph is $H$-free if it does not contain $H$ as a subgraph. The diamond graph is the graph obtained from $K_4$ by deleting one edge. We prove that if $G$ is a connected graph with order $n\geq 10$, then there exists a subset $S\subseteq V(G)$ with $|S|\leq n/5$ such that the graph induced by $V(G)\setminus N[S]$ is diamond-free, where $N[S]$ is the closed neighborhood of $S$. Furthermore, the bound is sharp.