The main share index fell 0.37 percent to 5,631.89, its lowest since Dec. 28.
Foreign investors, who mainly bought banks, acquired a net 858.3 million rupees worth of shares, reversing the year-to-date net foreign outflow to a net inflow of 319.2 million rupees.
Foreign buying accounted for 81.4 percent of turnover of 1.08 billion rupees ($8.48 million), which was just above this year's daily average of 1.05 billion rupees.
The market has fallen 3.5 percent in thin volume since the IMF on Feb. 13 said Sri Lanka was facing slow growth, high inflation and lower tax revenue risks.
Analysts said retailers were also worried about what could happen to Sri Lanka at UN Human Rights Council sessions in Geneva.
The rupee ended steady at 127.40/50 to the dollar in dull trade, but currency dealers said they expect depreciation pressure on the currency in coming weeks due to an expected rise in imports ahead of the April new year festival season. ($1 = 127.43 rupees)
(Reporting by Shihar Aneez; Editing by Nick Macfie)
(Reuters Messaging: email@example.com)