The main stock index ended up 0.6 percent, or 36.08 points, at 6,054.55 - its highest since Feb. 17.
Foreign selling accounted for 88.7 percent of the day's 5.04 billion rupees ($38.58 million) turnover, well above this year's daily average of 1.01 billion rupees.
The bourse saw a net foreign outflow of 4.08 billion rupees worth of shares extending the net foreign selling so far this year to 9.6 billion rupees.
Shares in Ceylon Tobacco rose 2.47 percent to 1,075 rupees, while shares in Sri Lanka Telecom Plc rose 2.72 percent to 49.10 rupees.
The top conglomerate John Keells Holdings Plc, which ended flat, saw a net foreign selling of 16.94 million shares.
Analysts, however, said investor sentiment is yet to recover after the United Nations announced it would probe alleged war crimes by the island nation.
The UN last week launched an inquiry into war crimes allegedly committed by both Sri Lankan state forces and Tamil rebels during a conflict that ended in 2009, saying the government had failed to investigate properly.
Analysts said the outcome of the resolution was expected, but investors' sentiment has been dented over concerns it could hurt the country's economy. Several potential buyers of risky assets are awaiting a clear direction. ($1 = 130.6500 Sri Lanka rupees) (Reporting by Ranga Sirilal and Shihar Aneez)