There is a lack of research which examines the effects of online pop-up dictionaries in the context of less commonly taught languages including Japanese as a second and foreign language (JSL and JFL), despite the growing popularity of such tools. This qualitatively -oriented study is an attempt to fill this void. The study investigates differences in how learners of Japanese at different proficiency levels read a target text when using an online pop-up dictionary. Participants’ think-aloud protocols were analysed to investigate their reading processes. A reading comprehension test examined their text understanding. A previous vocabulary knowledge test, a form recognition test, and a vocabulary translation test assessed participants’ vocabulary learning. A delayed vocabulary translation test examined how well the participants retained newly learned words. The overall findings demonstrate that learners of Japanese need to reach a certain proficiency level before they can strategically conduct online reading using pop-up dictionaries in order to deeply interact with their target texts. Regarding vocabulary learning, the results of the form recognition test as well as the immediate and delayed vocabulary translation tests suggest that this type of reading as a method of vocabulary learning can be beneficial.