Interword spacing facilitates English native readers but not native readers of Chinese, a writing system that does not mark word boundaries. L1-English readers of Chinese as a Second Language (CSL) could then be facilitated if spacing is added between words in Chinese materials. However, previous studies produced inconsistent results. This study tested the hypothesis that interword spacing facilitates L1-English CSL readers. We used an online multiple-choice gap-filling task to test 12 English CSL readers and 12 Chinese natives reading a series of eight texts of suitable difficulty, written with or without interword spacing. The CSL readers read faster with interword spacing than without, while Chinese native readers were not affected. The interword spacing effect was negatively correlated with measures of reading proficiency. It is argued that interword spacing facilitates CSL readers reading materials of sufficient difficulty by facilitating their lexical parsing. Pedagogical implications are discussed.