On Monday morning it topped the 40,000 level for the first time.
The index has hit new peaks after it last month beat the high set during Japan's bubble era economy three decades ago.
Tech stocks are driving the latest gains.
They've been boosted by the U.S. rally in AI-related companies like Nvidia.
That has pushed the S&P 500 and Nasdaq to record highs too.
Monday's big gainers in Japan included chipmaker Renesas, up over 5%.
Fellow chip tech firm Advantest rose over 3%.
The Nikkei itself was up close to 1% by late morning, putting it comfortably above the 40,000 mark.
It has now risen for five weeks straight.
Besides tech, it's been boosted by factors including a weak yen.
That has made Japanese stocks cheaper for overseas buyers.
Investors have also welcomed moves by the Tokyo Stock Exchange to pressure companies for better returns.
That has helped spur moves by many big firms to unwind cross-shareholdings.
The complex web of ties was seen as sheltering management from investor demands.
