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4-Coumarate: coenzyme A ligase (4CL; EC 6.2.1.12) is an important enzyme in the phenylpropanoid metabolic pathway that controls the biosynthesis of lignin and flavonoids. In this study, to identify the function of the <i>Ag4CL3</i> gene of celery, the <i>Ag4CL3</i> gene was cloned from celery cv. “Nanxuan Liuhe Ziqin”. Sequence analysis results showed that the <i>Ag4CL3</i> gene contained an open reading frame (ORF) with a length of 1688 bp, and 555 amino acids were encoded. The Ag4CL3 protein was highly conserved among different plant species. Phylogenetic analysis demonstrated that the 4CL proteins from celery and carrot belonged to the same clade. The Ag4CL3 protein was mainly composed of 31.89% α-helixes, 18.02% extended strands, 6.67% β-turns, and 43.42% random coils, and the signal peptide was unfound. A total of 62 phosphorylation sites and a class-I superfamily of adenylate-forming domains were found. As the growth time increased, the plant height and stem thickness also increased, and the petiole lignin content increased and became lignified gradually. The relative expression levels of the <i>Ag4CL3</i> gene in “Nanxuan Liuhe Ziqin” petioles were higher than those in other tissues, with the highest level occurring 70 d after sowing. The lignin contents in the transgenic <i>Arabidopsis thaliana</i> lines hosting the <i>Ag4CL3</i> gene were higher than those in the WT. In this study, the overexpression of <i>Ag4CL3</i> led to the significant upregulation of lignin biosynthesis gene expression in transgenic <i>A. thaliana</i> plants, except for <i>AtPAL, AtCCR,</i> and <i>AtLAC.</i> This study speculates that <i>Ag4CL3</i> genes are related to lignin synthesis in <i>A. graveolens.</i>