The Special Investigation Service of the Republic of Lithuania (commonly called STT; Lithuanian: Specialiųjų tyrimų tarnyba) is a law enforcement institution in Lithuania serving under the Seimas and the President of Lithuania to combat corruption. With departments in Kaunas, Klaipėda, Šiauliai and Panevėžys, it works to prevent corruption through education and investigates corruption where it exists. Since 2013, the director, appointed by a presidential decree, has been Saulius Urbanavičius.
With growing awareness of corruption in Lithuania after the 1990 Act of the Re-Establishment of the State of Lithuania, the Lithuanian government determined in 1997 to establish a special, separate organization for addressing the problem. The STT was accordingly established and for its first several years was overseen by the Ministry of the Interior. In May 2000, it came under the direct oversight of the presidency and Seimas. Since then, Lithuania has strengthened anti-corruption laws to enable the STT to better serve its purpose.