Empirical studies have evidenced the correlation between vocabulary knowledge and English-speaking skills in various English as a foreign language (EFL) contexts but research is scarce in rural Rwandan schools. The present study evaluated the impact of senior five (S5) learners’ vocabulary knowledge on developing their English-speaking skills in three selected schools in the Gakenke District in Rwanda. The current study was guided by Harmer’s and Bygate’s vocabulary knowledge hypothesis and communicative competence theories for developing speaking proficiency. A quantitative descriptive research design was employed and cluster sampling was used to select 90 S5 learners from 116 who made the target population. A structured questionnaire was administered to assess the current status of vocabulary knowledge and English-speaking skills and the impact of vocabulary knowledge on English-speaking skills. Descriptive statistics was used for data analysis using MS Excel. Key findings revealed that 73.3% of learners regularly look up new vocabulary, and 100% reported having insufficient vocabulary. Additionally, 84.44% indicated that their speaking proficiency was low to moderate. Despite this low vocabulary knowledge, 95.56% of learners acknowledged that vocabulary enhances speaking skills. However, only 15.5% reported using new vocabulary frequently. Many learners (81.11%) felt that vocabulary development improved their self-expression, and 54.44% noted increased confidence in speaking English. Motivation was the highest-ranked factor influencing speaking skill development (M=4.40, SD=0.85), while assessment ranked lowest (M=2.96, SD=0.97). The study concluded that S5 learners in the selected schools have insufficient vocabulary knowledge to achieve the required level of speaking proficiency. The study recommended reinforcing vocabulary acquisition instructions and opportunities for learners to practice communicative English and assigns that future work should emphasize formal tests that examine English proficiency in correlational and experimental studies.