Retail investors are actively discussing markets on social media, but measuring this sentiment is challenging. This paper presents the Reddit Retail Investor Sentiment Indicator (R-RISI), a new, high-frequency sentiment measure. R-RISI is derived from Reddit posts in investing and crypto subreddits, spanning from 2015 to April 2026. Large language models like ChatGPT 5.1 are used to classify post sentiment, proving more effective than finance-specific models for informal language. The indicator aggregates daily positive and negative posts, incorporating user engagement and a rolling z-score standardization. R-RISI can be tailored to various asset classes, portfolios, or individual securities, offering granular insights. The study demonstrates how sentiment and discussion topics evolve across different subreddits. R-RISI aligns with existing sentiment indicators but provides more frequent and timely market signals. Empirical evidence shows R-RISI changes contain significant short-term predictive information for market prices. This effect is amplified during market stress, particularly for next-day S&P 500 returns. R-RISI offers a valuable tool for understanding retail investor behavior and sentiment-driven market dynamics, with an accompanying online dashboard for regular updates.