Mit brennender Sorge (listen^ⓘ German pronunciation: [mɪt ˈbʀɛnəndɐ ˈzɔʁɡə], in English "With deep [lit. 'burning'] anxiety") is an encyclical of Pope Pius XI, issued during the Nazi era on 10 March 1937 (but bearing a date of Passion Sunday, 14 March). Written in German, not in the usual Latin, it was smuggled into Germany for fear of censorship and was read from the pulpits of all German Catholic churches on one of the Church's busiest Sundays, Palm Sunday (21 March that year).

The encyclical condemned breaches of the 1933 Reichskonkordat agreement signed between the German Reich and the Holy See. It condemned "pantheistic confusion", "neopaganism", "the so-called myth of race and blood", and the idolizing of the State. It contained a vigorous defense of the Old Testament with the belief that it prepares the way for the New. The encyclical states that race is a fundamental value of the human community, which is necessary and honorable but condemns the exaltation of race, or the people, or the state, above their standard value to an idolatrous level. The encyclical declares "that man as a person possesses rights he holds from God, and which any collectivity must protect against denial, suppression or neglect." National Socialism, Adolf Hitler and the Nazi Party are not named in the document. The German term Reichsregierung (government of the Reich) is used to refer to the Nazi regime.

The effort to produce and distribute over 300,000 copies of the letter was entirely secret, allowing priests across Germany to read the letter without interference. The Gestapo raided the churches the next day to confiscate all the copies they could find, and the presses that had printed the letter were closed. According to historian Ian Kershaw, an intensification of the general anti-church struggle began around April in response to the encyclical. Klaus Scholder wrote: "state officials and the Party reacted with anger and disapproval. Nevertheless the great reprisal that was feared did not come. The concordat remained in force and despite everything the intensification of the battle against the two churches which then began remained within ordinary limits." The regime further constrained the actions of the Church and harassed monks with staged prosecutions for alleged immorality and phony abuse trials. Though Hitler is not named in the encyclical, the German text does refer to a "Wahnprophet", which some have interpreted as meaning "mad prophet" and as referring to Hitler himself.

It was reported at the time that the encyclical was paired and somewhat overshadowed by the anti-communist encyclical Divini Redemptoris of 19 March, in order to avoid the charge that the Pope was indirectly favoring communism.

Following the Nazi takeover, the Catholic Church hierarchy in Germany initially attempted to co-operate with the new government, but by 1937 had become highly disillusioned. A threatening, though initially mainly sporadic persecution of the Catholic Church followed the Nazi takeover. Hitler moved quickly to eliminate Political Catholicism. Two thousand functionaries of the Bavarian People's Party were rounded up by police in late June 1933. They along with the national Catholic Centre Party, ceased to exist in early July, as the Nazi Party became the only legally permitted party (Gleichschaltung) in the country. Vice Chancellor Franz von Papen meanwhile negotiated the Reichskonkordat treaty with the Vatican, which prohibited clergy from participating in politics. Kershaw wrote that the Vatican was anxious to reach agreement with the new government, despite "continuing molestation of Catholic clergy, and other outrages committed by Nazi radicals against the Church and its organisations".

The Reichskonkordat (English: Reich Concordat) was signed on 20 July 1933 between the Holy See and Nazi Germany. According to historian Pinchas Lapide, the Nazis saw the treaty as giving them moral legitimacy and prestige, whilst the Catholic Church sought to protect itself from persecution through a signed agreement. According to Guenter Lewy, a common view within Church circles at the time was that Nazism would not last long, and the favorable Concordat terms would outlive the current regime (the Concordat does remain in force today). A Church handbook published with the recommendation of the entire German Church episcopate described the Concordat as "proof that two powers, totalitarian in their character, can find an agreement, if their domains are separate and if overlaps in jurisdiction become parallel or in a friendly manner lead them to make common cause". Lewy wrote "The harmonious co-operation anticipated at the time did not quite materialize" but that the reasons for this "lay less in the lack of readiness of the Church than in the short sighted policies of the Hitler regime."

In Mit brennender Sorge, Pope Pius XI said that the Holy See had signed the Concordat "in spite of many serious misgivings" and in the hope it might "safeguard the liberty of the church in her mission of salvation in Germany". The treaty comprised 34 articles and a supplementary protocol. Article 1 guaranteed "freedom of profession and public practice of the Catholic religion" and acknowledged the right of the church to regulate its own affairs. Within three months of the signing of the document, Cardinal Bertram, head of the German Catholic Bishops' Conference, was writing in a pastoral letter of "grievous and gnawing anxiety" with regard to the government's actions towards Catholic organisations, charitable institutions, youth groups, press, Catholic Action, and the mistreatment of Catholics for their political beliefs. According to Paul O'Shea, Hitler had a "blatant disregard" for the Concordat, and its signing was to him merely a first step in the "gradual suppression of the Catholic Church in Germany". Anton Gill wrote that "with his usual irresistible, bullying technique, Hitler then proceeded to take a mile where he had been given an inch" and closed all Catholic institutions whose functions weren't strictly religious:

It quickly became clear that [Hitler] intended to imprison the Catholics, as it were, in their own churches. They could celebrate mass and retain their rituals as much as they liked, but they could have nothing at all to do with German society otherwise. Catholic schools and newspapers were closed, and a propaganda campaign against the Catholics was launched.