In mathematics and physics, a Hamiltonian vector field on a symplectic manifold is a vector field defined for any energy function or Hamiltonian. Named after the physicist and mathematician Sir William Rowan Hamilton, a Hamiltonian vector field is a geometric manifestation of Hamilton's equations in classical mechanics. The integral curves of a Hamiltonian vector field represent solutions to the equations of motion in the Hamiltonian form. The diffeomorphisms of a symplectic manifold arising from the flow of a Hamiltonian vector field are known as canonical transformations in physics and (Hamiltonian) symplectomorphisms in mathematics.

Hamiltonian vector fields can be defined more generally on an arbitrary Poisson manifold. The Lie bracket of two Hamiltonian vector fields corresponding to functions ${\displaystyle f}$ and ${\displaystyle g}$ on the manifold is itself a Hamiltonian vector field, with the Hamiltonian given by the Poisson bracket of ${\displaystyle f}$ and ${\displaystyle g}$.

Suppose that ${\displaystyle (M,\omega )}$ is a symplectic manifold. Since the symplectic form ${\displaystyle \omega }$ is nondegenerate, it sets up a fiberwise-linear isomorphism

$${\displaystyle \omega :TM\to T^{*}M,}$$

between the tangent bundle ${\displaystyle TM}$ and the cotangent bundle ${\displaystyle T^{*}M}$, with the inverse

$${\displaystyle \Omega :T^{*}M\to TM,\quad \Omega =\omega ^{-1}.}$$

Therefore, one-forms on a symplectic manifold ${\displaystyle M}$ may be identified with vector fields and every differentiable function ${\displaystyle H:M\rightarrow \mathbb {R} }$ determines a unique vector field ${\displaystyle X_{H}}$, called the Hamiltonian vector field with the Hamiltonian ${\displaystyle H}$, by defining for every vector field ${\displaystyle Y}$ on ${\displaystyle M}$,

$${\displaystyle \mathrm {d} H(Y)=\omega (X_{H},Y).}$$Or more succinctly, ${\displaystyle \iota _{X_{H}}\omega =dH}$.