BEOL and MOL interconnect structures with a self-forming sidewall barrier layer are provided. In one aspect, a method of forming an interconnect structure includes: patterning a feature(s) in a dielectric; selectively forming a metal layer at a bottom of the at least one feature; depositing a liner layer lining the feature(s), wherein the conformal liner layer includes a metal alloy AB; depositing a metal onto the liner layer to form the interconnect structure; and annealing the interconnect structure under conditions sufficient to form a barrier layer including the component B along vertical sidewalls of the feature(s). A method of forming an interconnect structure including a via and a trench on top of the via is also provided, as is an interconnect structure.