The main share index closed 0.93 percent, or 54.34 points weaker, at 5,800.75, its lowest close since Jan 16.
'Foreigners are on the buying side and the market's fall was due to some large caps, which were sold in thin volume,' a stockbroker said on condition of anonymity.
Foreign investors were net buyers of 830.6 million rupees ($6.53 million) worth of shares, mainly acquiring shares in conglomerate John Keells Holdings, which gained 0.44 percent to 226 rupees.
That helped to reverse the year to date net foreign flow to a net inflow of 399.2 million rupees. Sri Lanka enjoyed a record foreign inflow of 38.63 billion rupees last year.
The day's turnover was 1.35 billion rupees, well above the daily average of 1.02 billion so far this year.
The rupee closed a tad stronger at 126.65/70 to the dollar from Monday's close of 127.10/15, on dollar inflows into government securities, traders said.
($1 = 127.1500 Sri Lanka rupees)
(Reporting by Ranga Sirilal and Shihar Aneez)
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