  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, $K\to \pi \nu \bar{\nu}$ 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 $B\to K^{(\ast)} \mu^+ \mu^-$ data. The complementarity with
collider constraints is discussed as well.
