The stock market news and analysis site was integrated into FT.com when Pearson took full control of the former joint venture site in October last year.
The site was originally launched as a joint venture with CBS-owned MarketWatch.com in 1999, when Pearson took at 34% stake in MarketWatch.com.
The deal gave MarketWatch.com, which operates the cbsmarketwatch.com website in the US, a 50% stake in FTMarketWatch, which Pearson bought back in October.
The buy-back comes as Pearson announced it was to reduce its £150m online expenditure -- 10% of its FT.com staff were axed in April.
The closer integration of FT Investor with the Financial Times will see the site change its web address to ft.com/investor.
Previous subscribers to FTMarketWatch will be able to use the same username and password for FT Investor, and a new "passport" service will be introduced in February to allow subscribers access to all areas of FT.com.
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