The main share index fell 1.01 percent, or 57.90 points, to close at 5,686.99, its lowest close since March 13.
It has fallen 2.5 percent in the last four sessions, mainly due to concerns over the rupee, which hit a record low last week.
The bourse was in oversold territory on Thursday, with the 14-day Relative Strength Index falling to 27.324, below its lower neutral level of 30, Thomson Reuters data showed.
Ceylinco Insurance shares fell 10.71 percent to 1052.60 rupees a share, while top conglomerate John Keells Holdings PLC fell 1.13 percent to 210.60 rupees.
The rupee traded weaker due to importer dollar demand and remained under pressure in the absence of strong dollar inflows.
The currency had hit an all-time low last week on panic buying by importers, but recovered later.
Thursday's turnover was 485.7 million rupees ($3.65 million), well below this year's daily average of about 893.6 million rupees.
Foreign investors were net sellers for the second straight session, selling 67.4 million rupees worth of shares, but they have been net buyers of 18.5 billion rupees in stocks so far this year.
($1 = 133.0000 Sri Lanka rupees)
(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Prateek Chatterjee)
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