We introduce the immersion poset $(\mathcal{P}(n), \leqslant_I)$ on partitions, defined by $λ\leqslant_I μ$ if and only if $s_μ(x_1, \ldots, x_N) - s_λ(x_1, \ldots, x_N)$ is monomial-positive. Relations in the immersion poset determine when irreducible polynomial representations of $GL_N(\mathbb{C})$ form an immersion pair, as defined by Prasad and Raghunathan (2022). We develop injections $\mathsf{SSYT}(λ, ν) \hookrightarrow \mathsf{SSYT}(μ, ν)$ on semistandard Young tableaux given constraints on the shape of $λ$, and present results on immersion relations among hook and two column partitions. The standard immersion poset $(\mathcal{P}(n), \leqslant_{std})$ is a refinement of the immersion poset, defined by $λ\leqslant_{std} μ$ if and only if $λ\leqslant_D μ$ in dominance order and $f^λ\leqslant f^μ$, where $f^ν$ is the number of standard Young tableaux of shape $ν$. We classify maximal elements of certain shapes in the standard immersion poset using the hook length formula. Finally, we prove Schur-positivity of power sum symmetric functions $p_{A_μ}$ on conjectured lower intervals in the immersion poset, addressing questions posed by Sundaram (2018).