Volatile secondary metabolites emitted by plants contribute to plant-plant, plant-fungus, and plant-insect interactions. The C(16)-homoterpene TMTT (for 4,8,12-trimethyltrideca-1,3,7,11-tetraene) is emitted after herbivore attack by a wide variety of plant species, including Arabidopsis thaliana, and is assumed to play a role in attracting predators or parasitoids of herbivores. TMTT has been suggested to be formed as a degradation product of the diterpene alcohol (E,E)-geranyllinalool. Here, we report the identification of Terpene Synthase 04 (TPS04; At1g61120) as a geranyllinalool synthase (GES). Recombinant TPS04/GES protein expressed in Escherichia coli catalyzes the formation of (E,E)-geranyllinalool from the substrate geranylgeranyl diphosphate. Transgenic Arabidopsis lines carrying T-DNA insertions in the TPS04 locus are deficient in (E,E)-geranyllinalool and TMTT synthesis, a phenotype that can be complemented by expressing the GES gene under the control of a heterologous promoter. GES transcription is upregulated under conditions that induce (E,E)-geranyllinalool and TMTT synthesis, including infestation of plants with larvae of the moth Plutella xylostella and treatment with the fungal peptide alamethicin or the octadecanoid mimic coronalon. Induction requires jasmonic acid but is independent from salicylic acid or ethylene. This study paves the ground to address the contribution of TMTT in ecological interactions and to elucidate the signaling network that regulates TMTT synthesis.