Due to immigration, globalization and a changing labor market, there is a recognized need for flexible learning. In Europe, second language (L2) acquisition specifically has become of vital importance. Research on less commonly taught languages (LCTL), however, is scarce and focuses mainly on foreign language acquisition, compulsory education and on face-to-face (F2F) and blended learning. However, the effectiveness of learning a LCTL -- such as Dutch -- as a second language fully online in adult education has yet to be uncovered. The principal objective of this paper was to investigate whether learning Dutch L2 fully online can be as effective as learning it fully F2F. A quantitative methodological approach was adopted. Independent samples t-tests were carried out to compare online and F2F adult learners' performance in a Waystage-level Dutch L2 course in Flanders, Belgium. The key variables 'previous knowledge', 'course level', 'content', 'teacher', 'assessment' and 'learning outcomes' are controlled. Main findings indicated that online Dutch L2 learning in adult education can be at least as effective as F2F learning. This study should, therefore, be of value to language course providers wishing to implement flexible L2 learning, and to LCTL and computer-assisted language learning (CALL) researchers.