The main stock index closed down 0.26 percent, or 15.64 points, at 5,922.23.
The index had ended Friday at its highest close since March 11.
Monday's turnover was 197.9 million rupees ($1.52 million) - less than a quarter of this year's daily average of 869.5 million rupees.
Foreign investors were net buyers of shares worth 12.6 million rupees.
Sri Lanka's 2.47 trillion rupee ($18.94 billion) bourse has seen a net outflow of 4.1 billion rupees so far this year. It reported a 22.88 billion rupee inflow in 2013.
Ceylon Tobacco shares fell 5.5 percent to 1,031.90 rupees, with only 35 shares traded.
Analysts said investor sentiment has been dented on concerns of a resolution that could hurt the country's economy.
Several potential buyers of risky assets are awaiting a clear direction.
Sri Lanka questioned the independence of the United Nations human rights office after the United States asked it to investigate violations by the government related to the civil war. A vote on the resolution is scheduled for later this week. ($1 = 130.6250 Sri Lanka Rupees) (Reporting by Shihar Aneez and Ranga Sirilal; Editing by Joyjeet Das)