Numerous games have served as testbeds for artificial intelligence (AI) research to measure its progress. Mahjong is a highly challenging multi-agent imperfect information game with a vast player population. However, a challenge with using Mahjong as a testbed for AI is the lack of a publicly available framework that is fast, easy to use and implements popular rules for human players. We propose and describe Mjx, an open-source Mahjong framework, which implements one of the most popular Mahjong rules, riichi Mahjong (Japanese Mahjong). We compared the execution speed of Mjx with existing popular open-source software and demonstrated that it achieves 100x faster performance. Mjx is available at https://github.conmjx-project/mjx.