In mathematics, a symplectomorphism or symplectic map is an isomorphism in the category of symplectic manifolds. In classical mechanics, a symplectomorphism represents a transformation of phase space that is volume-preserving and preserves the symplectic structure of phase space, and is called a canonical transformation.

A diffeomorphism between two symplectic manifolds ${\displaystyle f:(M,\omega )\rightarrow (N,\omega ')}$ is called a symplectomorphism if

where ${\displaystyle f^{*}}$ is the pullback of ${\displaystyle f}$. The symplectic diffeomorphisms from ${\displaystyle M}$ to ${\displaystyle M}$ are a (pseudo-)group, called the symplectomorphism group (see below).

The infinitesimal version of symplectomorphisms gives the symplectic vector fields. A vector field ${\displaystyle X\in \Gamma ^{\infty }(TM)}$ is called symplectic if

Also, ${\displaystyle X}$ is symplectic if the flow ${\displaystyle \phi _{t}:M\rightarrow M}$ of ${\displaystyle X}$ is a symplectomorphism for every ${\displaystyle t}$. These vector fields build a Lie subalgebra of ${\displaystyle \Gamma ^{\infty }(TM)}$. Here, ${\displaystyle \Gamma ^{\infty }(TM)}$ is the set of smooth vector fields on ${\displaystyle M}$, and ${\displaystyle {\mathcal {L}}_{X}}$ is the Lie derivative along the vector field ${\displaystyle X.}$

Examples of symplectomorphisms include the canonical transformations of classical mechanics and theoretical physics, the flow associated to any Hamiltonian function, the map on cotangent bundles induced by any diffeomorphism of manifolds, and the coadjoint action of an element of a Lie group on a coadjoint orbit.

Any smooth function on a symplectic manifold gives rise, by definition, to a Hamiltonian vector field and the set of all such vector fields form a subalgebra of the Lie algebra of symplectic vector fields. The integration of the flow of a symplectic vector field is a symplectomorphism. Since symplectomorphisms preserve the symplectic 2-form and hence the symplectic volume form, Liouville's theorem in Hamiltonian mechanics follows. Symplectomorphisms that arise from Hamiltonian vector fields are known as Hamiltonian symplectomorphisms.

Since {H, H} = X_H(H) = 0, the flow of a Hamiltonian vector field also preserves H. In physics this is interpreted as the law of conservation of energy.