Roots absorb water and nutrients from the soil, support the plant's aboveground organs, and detect environmental changes, making them crucial targets for improving crop productivity. Particularly sensitive to soil salinity, a major abiotic stress, roots face significant challenges that threaten global agriculture. In response to salt stress, plants suppress root meristem size, thereby reducing root growth. However, the mechanisms underlying this growth restriction remain unclear. Here, we investigate the role of reactive oxygen species (ROS) in this process and reveal that LATERAL ORGAN BOUNDARIES DOMAIN 11 (LBD11) plays a central role in ROS-mediated regulation of meristem size and the salt stress-induced inhibition of root growth. Under normal conditions, LBD11 controls the expression of key ROS metabolic genes, maintaining ROS homeostasis within root developmental zones to control meristem size and overall root growth. Upon sensing salt stress, LBD11 undergoes rapid proteasome-mediated degradation, leading to decreased distribution of O2⋅-, which in turn curtails meristem size and limits root length. Our findings highlight an unexplored plant adaptation strategy, where the growth-promoting LBD11/ROS pathway is downregulated to finely regulate root growth under challenging conditions. We propose a strategy for developing crops with heightened resilience and increased yields in salt-affected environments.