Lieutenant General Jagath Jayasuriya said the defeat of Tamil Tiger separatists last May meant more troops could be sent on UN missions.
Sri Lanka has had a deployment of 950 soldiers working with the UN in Haiti since 2004, when government troops were observing a truce with Tiger rebels in the island's northeast.
Despite the truce collapsing in early 2007, Sri Lanka maintained its peacekeeping mission in Haiti.
Government forces crushed the Tigers last May, ending Sri Lanka's 37-year separatist conflict.
"With the end of fighting (in Sri Lanka), we are able to commit more troops for peacekeeping duties and the increase to 5,000 men is the first step," Lieutenant General Jayasuriya said, according to his press office.
The general made the announcement while addressing troops at the northern town of Mankulam, an army official told AFP.
The army did not say where the additional troops would be deployed.
There was no immediate comment from the United Nations, wh