The community union took action on behalf of more than 1,100 of its members who lost their jobs when the site closed last year.
Workers who were part of Community's claim will now receive up to eight weeks' pay from the government's Redundancy Payments Office.
Roy Rickhuss, general secretary of Community, said: "This is a deserved victory for our SSI members and it is only right and proper that the tribunal found in our favour because of the way these workers were treated.
"As the lead union in the claim, the diligent preparation and hard work of our team of staff and lawyers has been rewarded today and our members will get what they are entitled to.”
Tom Blenkinsop, Labour MP for Middlesbrough South and East Cleveland, praised the union's campaigning, saying: “without the community's efforts on behalf of its members, and on behalf of other union members, the legal entitlements of these workers would never have been obtained.”