AbstractLexical proficiency in a second language (L2) has long been effectively assessed through the measurement of various lexical indices, or textual characteristics that act as observable indicators of such conceptual categories as lexical richness, diversity, sophistication, and fluency. While many studies have established links between these lexical characteristics and levels of overall language proficiency, languages other than L2 English rarely feature in such research. The current study addresses this gap by investigating a less commonly studied language, Russian, while paying specific attention to the operationalization of proficiency through an American Council for the Teaching of Foreign Languages Proficiency Guidelines‐based procedure. The study analyzes eight lexical proficiency indices in a corpus of essays written by learners of Russian at different proficiency levels. Our analyses demonstrate that seven indices (i.e., word length in letters, word length in syllables, word length in morphemes, unique tokens per text, unique lemmas per text, and measure of textual lexical diversity for tokens and for lemmas) changed significantly with the increase in proficiency. Only one index—the proportion of content versus function words—did not reliably track the increase in proficiency. The findings confirm the usefulness of lexical proficiency measures in tracking lexical development in L2 Russian and increase the repertoire of these indices for L2 Russian.