Halak, 28, was acquired by Washington on Mar. 5 from the Buffalo Sabres, who acquired him from the St. Louis Blues on Mar. 1.
The fourth-round pick Washington acquired originally belongs to the Chicago Blackhawks, who traded it to the Islanders on Feb. 6 for forwards Pierre-Marc Bouchard and Peter Regin.
Halak was set to become an unrestricted free agent July 1 and could have tested the waters if he didn't agree to a contract with the Islanders.
In 272 career NHL games with the Montreal Canadiens, St. Louis and Washington, Halak has a 144-85-29 record with a 2.38 goals-against average, a .918 save percentage and 30 shutouts.