  A graph $G$ is called $F$-saturated if $G$ does not contain $F$ as a subgraph
(not necessarily induced) but the addition of any missing edge to $G$ creates a
copy of $F$. The saturation number of $F$, denoted by $sat(n,F)$, is the
minimum number of edges in an $n$-vertex $F$-saturated graph. Determining the
saturation number of complete partite graphs is one of the most important
problems in the study of saturation number. The value of $sat(n,K_{2,2})$ was
shown to be $\lfloor\frac{3n-5}{2}\rfloor$ by Ollmann, and a shorter proof was
later given by Tuza. For $K_{2,3}$, there has been a series of study aiming to
determine $sat(n,K_{2,3})$ over the years. This was finally achieved by Chen
who confirmed a conjecture of Bohman, Fonoberova, and Pikhurko that $sat(n,
K_{2,3})= 2n-3$ for all $n\geq 5$. In this paper, we prove a conjecture of
Pikhurko and Schmitt that $sat(n, K_{3,3})=3n-9$ when $n \geq 9$.
