Sicherheitsdienst (German: [ˈzɪçɐhaɪtsˌdiːnst] ^ⓘ, "Security Service"), full title Sicherheitsdienst des Reichsführers-SS ("Security Service of the Reichsführer-SS"), or SD, was the intelligence agency of the SS and the Nazi Party in Nazi Germany. Established in 1931, the SD was the first Nazi intelligence organization and the Gestapo (formed in 1933) was considered its sister organization through the integration of SS members and operational procedures. The SD was administered as an independent SS office between 1933 and 1939. That year, the SD was transferred over to the Reich Security Main Office (Reichssicherheitshauptamt; RSHA), as one of its seven departments. Its first director, Reinhard Heydrich, intended for the SD to bring every single individual within the Third Reich's reach under "continuous supervision".

Following Germany's defeat in World War II, the tribunal at the Nuremberg trials officially declared that the SD was a criminal organisation, along with the rest of Heydrich's RSHA (including the Gestapo) both individually and as branches of the SS in the collective. Heydrich was assassinated in 1942; his successor, Ernst Kaltenbrunner, was convicted of war crimes and crimes against humanity at the Nuremberg trials, sentenced to death and hanged in 1946.

The SD, one of the oldest security organizations of the SS, was first formed in 1931 as the Ic-Dienst (Intelligence Service) operating out of a single apartment and reporting directly to Heinrich Himmler. Himmler appointed a former junior naval officer, Reinhard Heydrich, to organise the small agency. The office was renamed Sicherheitsdienst (SD) in the summer of 1932. The SD became more powerful after the Nazi Party took control of Germany in 1933 and the SS started infiltrating all leading positions of the security apparatus of the Reich. Even before Hitler became Chancellor in January 1933, the SD was a veritable "watchdog" over the SS and over members of the Nazi Party and played a critical role in consolidating political-police powers into the hands of Himmler and Heydrich.

Once Hitler was appointed Chancellor by German President Paul von Hindenburg, he quickly made efforts to manipulate the aging president. On 28 February 1933, Hitler convinced Hindenburg to declare a state of emergency which suspended all civil liberties throughout Germany, due at least in part to the Reichstag fire on the previous night. Hitler assured Hindenburg throughout that he was attempting to stabilize the tumultuous political scene in Germany by taking a "defensive measure against Communist acts of violence endangering the state". Wasting no time, Himmler set the SD in motion as they began creating an extensive card-index of the Nazi regime's political opponents, arresting labor organizers, socialists, Jewish leaders, journalists, and communists in the process, sending them to the newly established prison facility near Munich, Dachau. Himmler's SS and SD made their presence felt at once by helping rid the regime of its known political enemies and its perceived ones, as well. As far as Heydrich and Himmler were concerned, the SD left their mission somewhat vaguely defined so as to "remain an instrument for all eventualities". One such eventuality would soon arise.

For a while, the SS competed with the Sturmabteilung (SA) for influence within Germany. Himmler distrusted the SA and came to deplore the "rabble-rousing" brownshirts (despite once having been a member) and what he saw as indecent sexual deviants amid its leadership. At least one pretext to secure additional influence for Himmler's SS and Heydrich's SD in "protecting" Hitler and securing his absolute trust in their intelligence collection abilities, involved thwarting a plot from Ernst Röhm's SA using subversive means.

On 20 April 1934 Hermann Göring handed over control of the Geheime Staatspolizei (Gestapo) to Himmler. Heydrich, named chief of the Gestapo by Himmler on 22 April 1934, also continued as head of the SD. These events further extended Himmler's control of the security mechanism of the Reich, which by proxy also strengthened the surveillance power of Heydrich's SD, as both entities methodically infiltrated every police agency in Germany. Subsequently, the SD was made the sole "party information service" on 9 June 1934.

Under pressure from the Reichswehr (German armed forces) leadership (whose members viewed the enormous armed forces of the SA as an existential threat) and with the collusion of Göring, Joseph Goebbels, the Gestapo and SD, Hitler was led to believe that Röhm's SA posed a serious conspiratorial threat requiring a drastic and immediate solution. For its part, the SD provided fictitious information that there was an assassination plot on Hitler's life and that an SA putsch to assume power was imminent since the SA were allegedly amassing weapons. Additionally, reports were coming into the SD and Gestapo that the vulgarity of the SA's behavior was damaging the party and was even making antisemitism less palatable. On 30 June 1934 the SS and Gestapo acted in coordinated mass arrests that continued for two days. The SS took one of its most decisive steps in eliminating its competition for command of security within Germany and established itself firmly in the Nazi hierarchy, making the SS and its intelligence organ, the SD, responsible only to the Führer. The purge became known as the Night of the Long Knives, with up to 200 people killed in the action. Moreover, the brutal crushing of the SA and its leadership sent a clear message to everyone that opposition to Hitler's regime could be fatal. It struck fear across the Nazi leadership as to the tangible concern of the reach and influence of Himmler's intelligence collection and policing powers.

During the autumn of 1937, Hitler secured Mussolini's support to annex Austria (Mussolini was originally apprehensive of the Nazi takeover of Austria) and informed his generals of his intentions to invade both Austria and Czechoslovakia. Getting Mussolini to approve political intrigue against Austria was a major accomplishment, as the Italian Duce had expressed great concern previously in the wake of an Austrian SS unit's attempt to stage a coup not more than three weeks after the Röhm affair, an episode that embarrassed the SS, enraged Hitler, and ended in the assassination of Austrian Chancellor Engelbert Dollfuss on 25 July 1934. Nonetheless, to facilitate the incorporation of Austria into the greater Reich, the SD and Gestapo went to work arresting people immediately, using lists compiled by Heydrich. Heydrich's SD and Austrian SS members received financing from Berlin to harass Austrian Chancellor von Schuschnigg's government all throughout 1937. One section of the SD that was nothing more than a front for subversive activities against Austria ironically promoted "German-Austrian peace".