In mathematics, the Laplace transform, named after Pierre-Simon Laplace (), is an integral transform that converts a function of a real variable (usually ⁠${\displaystyle t}$⁠, in the time domain) to a function of a complex variable ${\displaystyle s}$ (in the complex-valued frequency domain, also known as s-domain or s-plane). The functions are often denoted using a lowercase symbol for the time-domain function and the corresponding uppercase symbol for the frequency-domain function, e.g. ${\displaystyle x(t)}$ and ⁠${\displaystyle X(s)}$⁠.

The transform is useful for converting differentiation and integration in the time domain into much easier multiplication and division in the Laplace domain (analogous to how logarithms are useful for simplifying multiplication and division into addition and subtraction). This gives the transform many applications in science and engineering, mostly as a tool for solving linear differential equations and dynamical systems by simplifying ordinary differential equations and integral equations into algebraic polynomial equations, and by simplifying convolution into multiplication.

For example, through the Laplace transform, the equation of the simple harmonic oscillator (Hooke's law) $${\displaystyle x''(t)+kx(t)=0}$$ is converted into the algebraic equation $${\displaystyle s^{2}X(s)-sx(0)-x'(0)+kX(s)=0,}$$ which incorporates the initial conditions ${\displaystyle x(0)}$ and ⁠${\displaystyle x'(0)}$⁠, and can be solved for the unknown function ${\displaystyle X(s).}$ Once solved, the inverse Laplace transform can be used to transform it to the original domain. This is often aided by referencing tables such as that given below.

The Laplace transform is defined (for suitable functions ⁠${\displaystyle f}$⁠) by the integral $${\displaystyle {\mathcal {L}}\{f\}(s)=\int _{0}^{\infty }f(t)e^{-st}\,dt,}$$ where s is a complex number.

The Laplace transform is related to many other transforms. It is essentially the same as the Mellin transform and is closely related to the Fourier transform. Unlike for the Fourier transform, the Laplace transform of a function is often an analytic function, meaning that it has a convergent power series, the coefficients of which represent the moments of the original function. Moreover, the techniques of complex analysis, especially contour integrals, can be used for simplifying calculations.