Japan’s stock-market index that peaked on December 29, 1989, was the Nikkei 225.
The Nikkei 225 reached an intraday high of 38,957.44 at the end of Japan’s asset-price bubble. The bubble had driven extraordinary increases in Japanese stocks and land prices during the second half of the 1980s.
After the peak, property and share prices collapsed. Japanese banks were left with large quantities of impaired loans, and the country entered a prolonged period of weak growth and financial repair often called the Lost Decades. The Nikkei’s 1989 peak remained unbroken for more than three decades.
The TOPIX is another important Japanese equity index, so it is a frequent alternative answer. The question specifically identifies the Nikkei 225, a price-weighted index of 225 major companies listed on the Tokyo Stock Exchange’s Prime Market.