Plant fatty acids (FAs) and lipids have essential functions in storing energy and act as structural components for cell membranes and signaling molecules for plant growth and stress responses. Acyl Carrier Proteins (ACPs) are small acidic proteins, which covalently bind the fatty acyl intermediates during the elongation of FAs. The Arabidopsis thaliana ACP family has 8 members. Through reverse genetic, molecular, and biochemical approaches, we have discovered that ACP1 localizes to the chloroplast and limits the magnitude of pattern-triggered immunity (PTI) against the bacterial pathogen Pseudomonas syringae pathovar tomato (Pto). The mutant acp1 plants have reduced the levels of linolenic acid (18:3), which is the main precursor for the biosynthesis of the phytohormone jasmonic acid (JA), and a corresponding decrease in the abundance of JA. Consistent with the known antagonistic relationship between JA and salicylic acid (SA), acp1 plants also over-accumulate SA and display the corresponding shifts in JA- and SA-regulated transcriptional outputs. The ability of ACP1 to prevent this hormone imbalance likely underlies its negative impact on PTI in plant defense. Thus, ACP1 links FA metabolism to stress hormone homeostasis to be negatively involved in PTI in Arabidopsis plant defense.