Let $E$ be a set of $n$ elements, $f\colon2^E\to\mathbb Z_{\ge0}$ be submodular, $t_1,\ldots,t_r\in E$ be distinct, and $k\in\mathbb N$. The Submodular Multiway Partition problem asks whether $E$ has a partition $(P_1,\ldots,P_r)$ with $t_i\in P_i$ for all $i$ and $\sum_i f(P_i)\le k$. We prove that this problem can be solved in $r^{O(k)}n^{O(1)}$ time whenever $f$ is posimodular. This covers monotone submodular functions, matroid rank functions, and nonnegative combinations of monotone and symmetric submodular functions. When $f$ is a connectivity function, we remove the dependence on the number of terminals and obtain a $k^{O(k)}n^{O(1)}$-time algorithm. The proof is entirely elementary, and is arguably the simplest proof that Edge Multiway Cut is fixed-parameter tractable. As a new consequence, our main result immediately implies that Hypergraph Multiway Partition is fixed-parameter tractable.

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