The Nikkei 225 reached its all-time peak on December 29, 1989, before Japan’s market crash.
The index closed at 38,915.87 that day, near the end of Japan’s asset-price bubble. During the late 1980s, Japanese share and land prices rose dramatically, supported by easy credit and optimistic expectations. When monetary policy tightened and the bubble burst, Japanese equities entered a long decline.
The Nikkei 225 is a price-weighted index of 225 major companies listed on the Tokyo Stock Exchange’s Prime Market. Because it is price-weighted, a company’s share price affects the index more directly than its total market capitalization does. That structure distinguishes it from many widely followed capitalization-weighted indexes.
The 1989 peak is often used as a symbol of the beginning of Japan’s “Lost Decades,” although the economic consequences unfolded over years. A related index, the TOPIX, measures a broader set of Tokyo-listed companies and is therefore not the same answer.