The principal stock index that collapsed during Japan's 1989 asset-price bubble crash was the Nikkei 225.
The Nikkei 225 reached an intraday record of 38,957.44 on December 29, 1989, near the end of Japan's extraordinary property and equity boom. The bubble had been fueled by rapid credit expansion, rising land values, and strong investor confidence. After monetary tightening and falling asset prices, Japanese equities entered a prolonged decline.
The Nikkei is a price-weighted index of 225 prominent companies listed on the Tokyo Stock Exchange's Prime Market. Its collapse was part of a wider financial and economic downturn often called Japan's Lost Decades.
TOPIX is another major Japanese index, but it is capitalization-weighted and is not the index named in this question. The bubble's stock-market peak is commonly dated to the end of 1989, even though the broader economic effects continued for years.