The Colombo Stock Exchange's main index ended 0.41 percent or 22.22 points, weaker at 5,384.89.
Markets will be closed for a public holiday on Tuesday and stockbrokers said many investors were on an extended break.
"The high interest rate still weighs on the market. When fixed deposits give a return of around 17-18 percent, investors will hardly come to the stock market," said a stockbroker who declined to be identified.
Analysts said investors had been moving in to fixed deposits from equities after the central bank kept key policy rates at three-year highs after the market closed on Friday.
The yield on the 364-day T-bill gained for the sixth consecutive week at the last weekly auction last Wednesday, central bank data showed.
The day's turnover was 116 million rupees ($890,900), its lowest since July 2 and well below the year's daily average of 906 million rupees. Foreign investors bought a net 28.9 million rupees of shares, extending the net foreign inflow this year to 34.55 billion rupees.
The rupee changed little to close at 130.10/20 to the dollar compared with Friday's close of 130.10/18 in dull trade, dealers said.
($1 = 130.2000 Sri Lanka rupees)
(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Robert Birsel)
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