We prove that the ∞{\infty}-category of motivic spectra satisfies Milnor excision: if A→B{A\to B} is a morphism of commutative rings sending an ideal IA{I\subset A} isomorphically onto an ideal of B, then a motivic spectrum over A is equivalent to a pair of motivic spectra over B and A/I{A/I} that are identified over B/IB{B/IB}. Consequently, any cohomology theory represented by a motivic spectrum satisfies Milnor excision. We also prove Milnor excision for Ayoub's étale motives over schemes of finite virtual cohomological dimension.