The State Security Department (SSD) of the Republic of Lithuania is a state institution accountable to the Seimas (Parliament) and the President, charged with the protection of the sovereignty of the Republic of Lithuania and its constitutional system.

The role of the Department is to identify activities which constitute a threat to the security of the state, its territorial inviolability and integrity, the interests of the state, and its economic and defence potential. The SSD fights against such activities and investigates the causes of their appearance and conditions, with means provided by the law.