Abstract: Retail investors increasingly discuss markets in real time on social media, yet these discussions remain difficult to measure systematically. This paper introduces the Reddit Retail Investor Sentiment Indicator (R-RISI), a high-frequency measure of retail investor sentiment based on Reddit posts from major investing and crypto-related subreddits between 2015 and April 2026. Using ChatGPT 5.1, we classify posts by sentiment and topic and show that large language models provide more accurate labels for informal social media language than widely used finance-specific models such as FinBERT, which tends to over-assign neutral sentiment. R-RISI is constructed by aggregating daily positive and negative posts, weighting them by user engagement and standardising the resulting series using a rolling one-year z-score methodology. The indicator can be flexibly built for broad asset classes, portfolios, individual securities or thematic groups, allowing sentiment to be tracked at a much higher granularity than traditional retail investor indicators. We show how sentiment and dominant discussion topics evolve over time within each of the extracted subreddits. R-RISI closely reflects major market developments and aligns with established sentiment indicators while providing higher frequency and more timely signals. Empirically, changes in R-RISI contain statistically significant short-term information for market prices. This relationship is particularly relevant during periods of market stress: changes in R-RISI matter more for next-day S&P 500 returns when markets are in decline. The indicator therefore offers a timely and granular tool for analysing retail investor behaviour and monitoring sentiment-driven market dynamics in an environment of growing retail investor participation. A regularly updated version of the indicator is available through an accompanying R-RISI online dashboard.