From the very beginning of his scientific career as a mathematician and astronomer, Carl Friedrich Gauß had close relations with the Russian Imperial Academy of Sciences in St. Petersburg and with a number of colleagues working at the universities of Kasan and Dorpat. At the age of 62 he began to learn Russian and study its structure, above all in order to train his memory and to relax from solving mathematical problems. The present article outlines the different stages of his learning progress and tries to determine the (obviously quite high) degree to which Gauß mastered this language that up to then had been completely unknown to him. This is done by analyzing the way Gauß used different textbooks and grammars of Russian and by estimating the amount of his reading matter. The paper also points out that Gauß, in all probability, was the first to device a reverse Russian glossary.