The main stock index ended 0.32 percent, or 22.50 points, lower at 7,031.33.
Turnover stood at 628.1 million rupees ($4.7 million), well below this year's daily average of about 1.1 billion rupees.
"The market continued its downtrend, ending the day on a negative note, with institutional and high networth investors dominant," TKS securities said in a note to investors.
Nestle Lanka fell 2.40 percent, while conglomerate John Keells fell 0.49 percent.
Keells said in a disclosure to the bourse that its subsidiary Waterfront Properties (Private) Ltd had finalised a $395-million syndicated project development facility with Standard Chartered Bank for debt financing of its luxury real estate project.
The market saw net foreign outflow of 166.2 million rupees on Thursday, extending net foreign outflows in the past 22 sessions to 3.63 billion rupees in stocks.
Investors were confused due to a lack of direction on interest rates, economic policies, and on the timing of the parliamentary election, analysts said.
President Maithripala Sirisena has said he will dissolve the parliament once some crucial reforms, including an electoral bill, are passed, but is yet to fix a date for the election.