The paper's main aim consisted in distinguishing and bringing together language learning strategies of the particular learner group of multilingual adults learning additional languages. Therefore, research was placed in the intersection of the three fields: language learning strategies (LLS), third language acquisition (TLA), and the didactics of plurilingualism. First, the paper synthesises the main findings of research on LLS applied by multilinguals learning additional (European) languages. Then two interconnected empirical studies combining quantitative and qualitative research methods are reported. The strategies distinguished in the literature and in the mixed-method study 1 (n = 18) were integrated into Oxford’s [(1990). Language learning strategies: What every teacher should know. Boston, MA: Heinle and Heinle] taxonomy and into a new strategy questionnaire for multilingual learners (Multilingual Strategy Inventory for Language Learning (M-SILL)). Almost 90% of the strategies tested in the large-scale survey study 2 (n = 271) were reported by the multilingual participants as applied with a high or moderate frequency. A significant correlation was detected between the learners’ degree of multilingualism and the use of the M-SILL strategies, with a major increase in L3 learning. This finding supplied an additional support to the claims that there is a ‘threshold’ effect in L3 learning and that TLA is different from SLA.