The main share index ended down 0.3 percent, or 18.26 points, at 6,037.22, its lowest close since July 25.
John Keells Holdings Plc, which reported a 4 percent drop in June quarter profit on Friday, fell 1.40 percent to 260.40 rupees a share.
Foreign investors were net sellers of 461 million rupees ($3.50 million) worth of shares, extending this week's net outflow to 649 million rupees.
But they have been net buyers of 16.3 billion rupees of shares so far this year.
Analysts said despite the central bank's easing of policy measures, concerns over the weakening rupee and high lending rates had dented sentiment, even as investors waited for direction.
The turnover was 906.3 million rupees, the highest since June 27, but less than this year's daily average of about 937 million rupees.
($1 = 131.6500 Sri Lanka rupees)
(Reporting by Ranga Sirilal; Editing by Jijo Jacob)