The study identifies the Arabidopsis thaliana gene *ALR1*, a leucine-rich repeat receptor-like kinase (LRR-RLK), as a plant aluminum ion (Al) sensor. Al ions are highly toxic to plants and cause significant crop yield losses and forest decline on acidic soils. The *alr1-1* mutant, which lacks *ALR1*, is highly sensitive to Al, exhibiting reduced root growth and increased Al content in roots and shoots. Overexpression of *ALR1* in the *alr1-1* mutant restores Al resistance. ALR1 binds Al ions specifically in its cytoplasmic domain, recruiting the co-receptor BAK1 and promoting the phosphorylation of the NADPH oxidase RbohD. This phosphorylation increases reactive oxygen species (ROS) production, which oxidatively modifies the RAE1 F-box protein. Oxidative modification of RAE1 inhibits its proteolytic activity, leading to increased accumulation of the central regulator STOP1. STOP1 activates the expression of genes involved in organic acid anion secretion, which chelates Al ions and confers Al resistance. The study also reveals that ALR1-mediated Al perception is independent of its PSK sensing function and that ALR1 is an Al ion receptor, with specific Cys residues in its cytoplasmic domain being essential for Al binding. These findings provide insights into ion-sensing mechanisms in plants and offer potential for molecular breeding of acid-soil-tolerant crops and trees, enhancing global food security and forest restoration.The study identifies the Arabidopsis thaliana gene *ALR1*, a leucine-rich repeat receptor-like kinase (LRR-RLK), as a plant aluminum ion (Al) sensor. Al ions are highly toxic to plants and cause significant crop yield losses and forest decline on acidic soils. The *alr1-1* mutant, which lacks *ALR1*, is highly sensitive to Al, exhibiting reduced root growth and increased Al content in roots and shoots. Overexpression of *ALR1* in the *alr1-1* mutant restores Al resistance. ALR1 binds Al ions specifically in its cytoplasmic domain, recruiting the co-receptor BAK1 and promoting the phosphorylation of the NADPH oxidase RbohD. This phosphorylation increases reactive oxygen species (ROS) production, which oxidatively modifies the RAE1 F-box protein. Oxidative modification of RAE1 inhibits its proteolytic activity, leading to increased accumulation of the central regulator STOP1. STOP1 activates the expression of genes involved in organic acid anion secretion, which chelates Al ions and confers Al resistance. The study also reveals that ALR1-mediated Al perception is independent of its PSK sensing function and that ALR1 is an Al ion receptor, with specific Cys residues in its cytoplasmic domain being essential for Al binding. These findings provide insights into ion-sensing mechanisms in plants and offer potential for molecular breeding of acid-soil-tolerant crops and trees, enhancing global food security and forest restoration.