Purpose: Findings from previous speech production research suggest that the nature of phonological planning units is language-specific, with phonemes as the planning units in Dutch and English but syllables in Cantonese and Mandarin Chinese. However, little is known about how multilingual speakers possessing languages with distinctive phonological planning units plan for their speech. This study was conducted to investigate the roles of syllable and phoneme in Cantonese (L1) and English (L2) production among Cantonese–English bilinguals. Design: A picture version of the form-preparation task was employed, where participants named aloud (in their L1 and L2) small sets of pictures repeatedly. The names of the pictures in a set either shared the same word-initial syllable, word-initial phoneme, or were unrelated. Participants’ L1 and L2 proficiencies were measured. Data and analysis: 53 Cantonese–English bilinguals participated. The naming latency data were submitted for linear mixed effects modeling analyses. Findings: Significant priming effects on naming latency, relative to an unrelated control, were found when the response words shared the same word-initial syllable but null effects were found when they shared merely the same word-initial phoneme in both L1 and L2 production. Critically, in L2 English production, a significant negative association was observed between the size of syllable priming and the participant’s English vocabulary size. Furthermore, significant English phonemic priming was observed among the participants higher in spoken English proficiency but not among the lower proficiency counterparts. Originality: This study provides clear evidence showing the multifaceted effects of L2 proficiency on L2 phonological planning using a picture naming task. Significance: The present findings indicate the significant impact of L1 on L2 phonological planning. Yet, this L1-to-L2 influence would be modulated in different ways by the different aspects of one’s L2 proficiency.