Organic fertilizers based on seaweed extract potentially have beneficial effects on many crop plants. Here we investigate the impact of organic fertilizer on Rosmarinus officinalis measured by both yield and oil quality. Plants grown in a temperature-controlled greenhouse with a natural photoperiod and a controlled irrigation system were treated with seaweed fertilizer and an inorganic fertilizer of matching mineral composition but with no organic content. Treatments were either by spraying on to the foliage or watering direct to the compost. The essential oil was extracted by hydro-distillation with a Clevenger apparatus and analysed by gas-chromatography mass-spectrometry (GC–MS) and NMR. The chemical compositions of the plants were compared, and qualitative differences were found between fertilizer treatments and application methods. Thus sprayed seaweed fertilizer showed a significantly higher percentage of β-pinene, α-phellandrene, α-terpinene (monoterpenes) and 3-methylenecycloheptene than other treatments. Italicene, α-bisabolol (sesquiterpenes), α-thujene, and E-isocitral (monoterpenes) occurred in significantly higher percentages for plants watered with the seaweed extract. Each was significantly different to the inorganic fertilizer and to controls. The seaweed treatments caused a significant increase in oil amount and leaf area as compared with both inorganic treatments and the control regardless of application method.