This study investigates the impact of thesauruses on vocabulary acquisition and writing proficiency among language learners and writers. Thesauruses are commonly utilized tools that provide users with synonyms and antonyms, aiding in the expansion of vocabulary and enhancement of writing skills. An experimental design was employed, involving 250 participants-125 language learners and 125 writers-who engaged with thesauruses over a three-week period. Quantitative measures included pre- and post-test assessments to evaluate vocabulary knowledge and writing quality, while qualitative data were collected through participant interviews and reflective journals. The findings indicate that regular use of thesauruses leads to significant improvements in vocabulary retention, lexical diversity, and overall writing sophistication. Participants reported increased confidence in their word choices and a greater understanding of word relationships. However, challenges such as misinterpretation of nuanced meanings and the risk of over-reliance on thesaurus suggestions were noted, highlighting the necessity for contextual awareness.