This study examined stress processing among Mandarin and Korean second language learners of English and English monolinguals. While both English and Mandarin have contrastive stress at the word-level, Korean does not. Consequently, Mandarin speakers may have an advantage over Korean speakers in English stress processing, even when matched for their general English proficiency. Experiment 1 assessed participants’ stress encoding ability for nonwords in a short-term memory task. Experiment 2 examined the effect of stress in online word recognition in a lexical decision task by manipulating word frequency, stress location, and vowel quality. The results of both experiments support an advantage for English and Mandarin speakers over Korean speakers in stress processing of real words and nonwords. Only Korean speakers’ lexical judgment of nonwords was modulated by word frequency, suggesting that they do not utilize stress in lexical access. Only English speakers’ word recognition was facilitated by vowel quality changes. These results suggest that the abilities of non-native speakers to process stress in their L2 is influenced by the characteristics of the stress systems in their L1.