Thymus vulgaris is an important medicinal plant across the world that has been grown in Iran since ancient times. To study the effect of foliar application of salicylic acid (SA) on the yield and phytochemical compositions of T. vulgaris, an experiment was conducted in Qazvin in 2017. A factorial experiment based on a randomised complete block design was conducted with three replications. Treatments of SA (0, 100, 150 and 200 mg/L) were applied from the period before flowering until the early stages of flowering under well-watered and withholding irrigation conditions. The results showed that SA had a significant effect on the yield and phytochemical compositions of T. vulgaris under both irrigation regimes. SA foliar application led to an increase in total dry matter. The highest total dry matter (1958.83 kg ha-1) was obtained by applying 200 mg/LSA under normal irrigation. The effect of foliar application of SA on the essential oil content of T. vulgaris was influenced by the irrigation regime. The highest essential oil content (2.66%) was obtained by applying a foliar spray of 100 mg/LSA under withholding irrigation conditions. Thymol, carvacrol, p-cymene, linalool and γ-terpinene were identified as the main essential oil compounds. The highest thymol content was obtained by foliar application of 200 mg/LSA under withholding irrigation conditions. Our results demonstrate that foliar application of SA reduced the negative effect of water deficit on T. vulgaris and increased yield and thymol content. SA foliar application is a simple, eco-friendly and relatively commercially viable method of increasing yield and the synthesis of phytochemicals; therefore, it may be used to improve the dry matter and quality of the essential oils of thyme plants.