The main share index ended 0.42 percent, or 25.30 points, weaker at 5,982.35, its lowest since July 9.
The index hit a near 10-week low on July 9 on concerns over the rupee's weakening trend and possible foreign outflows.
Turnover was 191.7 million rupees ($1.46 million), provisional data showed, the lowest since Dec. 24 and well below this year's daily average of 964.4 million rupees.
Shares in oil palm cultivator Good Hope Plc fell 21.42 percent to 1,100 rupees in thin trade, while leading mobile phone operator Dialog Axiata fell 2.3 percent to 8.50 rupees.
The local currency weakened to a 10-month low early on
Wednesday due to importer demand for dollar and there was no central bank intervention.
($1 = 131.4000 Sri Lanka rupees)
(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Subhranshu Sahu)