We explore the signals of axion-like particles (ALPs) in flavor-changing neutral current (FCNC) processes. The most general effective linear Lagrangian for ALP couplings to the electroweak bosonic sector is considered, and its contribution to FCNC decays is computed up to one-loop order. The interplay between the different couplings opens new territory for experimental exploration, as analyzed here in the ALP mass range $0<m_a lesssim 5$ GeV. When kinematically allowed, $Kto pi u bar{ u}$ decays provide the most stringent constraints for channels with invisible final states, while $B$-meson decays are more constraining for visible decay channels, such as displaced vertices in $Bto K^{(ast)} mu^+ mu^-$ data. The complementarity with collider constraints is discussed as well.
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