This study examines the speaking confidence of 11th grade students with high vocabulary mastery at a high school in Sragen, Central Java. Despite high Vocabulary Levels Test (VLT) scores, many students exhibited hesitation and anxiety when speaking English. This study addresses two research questions: (1) How do students with high vocabulary mastery perceive and describe their confidence in speaking English? and (2) What factors influence students’ speaking confidence with high vocabulary mastery when speaking English? A qualitative case study design was employed with six purposively selected participants based on a minimum VLT score of 80% on both word frequency levels. Data were collected through in-depth interviews, classroom observation, and document analysis, and analyzed using reflexive thematic analysis. The findings reveal two overarching results: RQ1: (a) speaking confidence is neither uniform nor automatically derived from vocabulary knowledge; (b) vocabulary perceived as identity marker produced more stable confidence; (c) a mismatch was found between receptive vocabulary knowledge and speaking readiness; and (d) confidence developed through accumulated speaking experiences with preparation dependency as a transitional mechanism. RQ2: (a) non-judgmental peer climate was the most critical condition for speaking willingness; (b) peer support functioned as affective and vocabulary scaffold; and (c) six factors were identified: fear of judgment, vocabulary-confidence mismatch, situational variability, audience size, social comparison, and grammar anxiety, forming a self-reinforcing anxiety cycle. This study concludes that vocabulary mastery is necessary but insufficient for speaking confidence, proposing confident productive deployment as a new dimension of vocabulary knowledge with implications for EFL instruction.