Event study is a widespread technique for testing the semi-strong form of the market efficiency hypothesis. Among traditionally studied events, changes in corporate credit ratings by rating agencies have a special importance, since rating agencies use both publicly available and insider information. Studies of developed and emerging markets point to different reactions of stock prices to rating upgrades and downgrades and identify several factors affecting the scale of this effect, including the size and liquidity of the stock market, the level of regulation of the industry, the market capitalization of the company, the status of the rating agency, and others.   On the Russian market, the impact of credit rating upgrades and downgrades on stock prices has not been investigated so far. Ongoing studies of other events affecting stock prices show that the market’s reactions are pretty much in line with those of developed markets, despite its immaturity, limited transparency, high volatility, narrowness and low liquidity, as well as the small number of significant events.   In this article, we evaluate the level of efficiency of the Russian stock market and analyse the reaction of stock prices to changes in issuer credit ratings by international rating agencies using the traditional event study methodology in a narrow event window of 31 days over the period 2016–2020 on a sample of 49 public companies. We show that credit rating upgrades do not lead to statistically significant positive abnormal returns. Visual analysis demonstrates that rating downgrades result in substantial negative abnormal returns. This effect varies for financial and non-financial companies and companies with low and high capitalization yet differs from the effects observed for developed markets; nevertheless, these abnormal returns are not statistically significant. Still, there are grounds to conclude that the Russian stock market is not efficient in the semi-strong form and is closer in its characteristics to emerging markets, which is important information for investors, as it permits them to develop profitable trading strategies.