Let $Γ= (G, σ)$ be a signed graph, where $G = (V(G),E(G))$ is an (unsigned) graph, called the underlying graph. The net Laplacian matrix of $Γ$ is defined as $L^{\pm}(Γ) = D^{\pm}(Γ) - A(Γ)$, where $D^{\pm}(Γ)$ and $A(Γ)$ are the diagonal matrix of net-degrees and the adjacency matrix of $Γ$, respectively. The nullity of $L^{\pm}(Γ)$, written as $ η(L^{\pm} (Γ))$, is the multiplicity of 0 as an eigenvalue of $L^{\pm}(Γ)$. In this paper, we focus our attention on the nullity of the net Laplacian matrix of a connected signed graph $Γ$ and prove that $1 \leq η(L^{\pm} (Γ)) \leq min\{ β(Γ) + 1, |V(Γ)| - 1 \}$, where $β(Γ) = |E(Γ)| - |V(Γ)| + 1$ is the cyclomatic number of $Γ$. The connected signed graphs with nullity $|V(Γ)| - 1$ are completely determined. Moreover, we characterize the signed cactus graphs with nullity $1$ or $β(Γ) + 1$