Organic chemical priming is an effective strategy in mitigating salt stress to plants. The objective of this study was to determine effects and associated mechanisms of acetic acid regulating plant tolerance to salt stress. Perennial ryegrass plants were pre-treated with 20 mM acetic acid and subsequently subjected to salt stress for 28 days. Salt stress caused increased endogenous acetic acid content with up-regulated expression of its key biosynthetic gene LpPDC1. Application of acetic acid effectively alleviated salt caused damage in perennial ryegrass. Acetic acid treatment increased K+ content and suppressed Na+ accumulation to maintain a higher K+/Na+ ratio in leaves exposed to salt stress. Plants treated with acetic acid also had significantly lower levels of and H 2 O 2, but higher SOD and CAT activities than those of the control after 21 days of salt stress. Acetic acid treatment also altered the plants endogenous phytohormone content with higher content of jasmonate (including JA, JA-ILe, and cis-OPDA), auxin (IAA), and cytokinins (CK, such as tZ, cZR, and iP), but lower content of abscisic acid (ABA) under salt stress conditions. Furthermore, expression of genes involved in JA, IAA, and CK biosynthesis and signaling pathways were up-regulated, while those involved in ABA were down-regulated by acetic acid treatment under salt stress. The results demonstrate that acetic acid could mitigate salt stress in perennial ryegrass by regulating K+ and Na+ balance, promoting ROS scavenging, and activating stress-protection hormone synthesis and signaling.