The market has been awaiting signs of political stability after President Maithripala Sirisena suspended parliament until May 8. A cabinet reshuffle was expected on Monday, but government sources said it had been delayed.
The Colombo stock index ended 0.13 percent weaker at 6,516.71, its lowest close since April 16. The index gained 0.88 percent last week.
Turnover stood at 356.8 million rupees ($2.3 million), less than a third of this year's daily average of 1.2 billion rupees.
Shares in Ceylon Tobacco Company Plc fell 1 percent, while Distilleries Company of Sri Lanka Plc closed 2.3 percent down and Softlogic Holdings Plc ended 4.0 percent weaker.
Foreign investors sold shares worth a net 25.8 million rupees on Tuesday, but they have been net sellers of 1.2 billion rupees worth of equities so far this year.