Psalm 126 is the 126th psalm of the Book of Psalms, beginning in English in the King James Version: "When the Lord turned again the captivity of Zion, we were like them that dream". In Latin, it is known as In convertendo Dominus. It is one of the fifteen Songs of Ascent in the Book of Psalms whose opening words in Hebrew are "Shir HaMaalot" (שיר המעלות בשוב ה’, a Song of Ascents). In the slightly different numbering system used in the Greek Septuagint version of the Bible and in the Latin Vulgate, this psalm is Psalm 125.

This six-verse psalm forms a regular part of Jewish, Catholic, Lutheran, Anglican and other Protestant liturgies. It is well known in Judaism as the preliminary psalm recited before the Birkat Hamazon (Grace After Meals) on Shabbat and Jewish holidays, and as such is sung to a wide variety of melodies. It has also inspired hymns based on it, and has often been set to music, such as Jean-Philippe Rameau and Jules Van Nuffel who set the psalm in Latin.

Parts of this psalm have been singled out, for example They that sow in tears shall reap in joy is included in Ein deutsches Requiem by Johannes Brahms.

Psalm 126 expresses the themes of redemption and joy and gratitude to God. According to Matthew Henry, it was likely written upon the return of the Israelites from Babylonian captivity. In Henry's view, the psalm was written either by Ezra, who led the nation at that time, or by one of the Jewish prophets. Jewish scholarship pairs this psalm with Psalm 137, with Psalm 137 commemorating the beginning of the Babylonian exile, and Psalm 126 describing the end of that exile. The grammatical structure of the psalm, however, suggests that it is talking both about a past redemption (from Babylonian captivity, in verse 1) and a future redemption (the permanent return of the exiles at the end of days, in verse 4). Alternately, modern Jewish commentators suggest that the second half of the psalm refers to the redemption of the land of Israel from agricultural drought.

The Talmud (Ta'anit 23a) mentions this psalm in the context of the famous story of Honi ha-M'agel, who slept for seventy years. Before he fell asleep, Honi saw an old man planting a carob tree that would not bear fruit for seventy years. The Talmud begins,

Rabbi Yohanan said: This righteous man [Honi HaMa'agel] was troubled throughout the whole of his life about the meaning of the verse from Psalms 126, "A Song of Ascents, When the Lord brought back those that returned to Zion, we will be like dreamers." He wondered: Is it possible for one man to live long enough to dream continuously for seventy years? [As it is written, "For the Lord said: When Babylon's seventy years are over, I will take note of and I will fulfill you to my promise of favor -- to bring you back to this place" (Jeremiah 29:10).]

Honi then ate a meal and fell asleep for seventy years. When he awoke, he saw the man's grandson gathering the fruits of the carob tree. Honi returned to the study hall and heard scholars discussing laws that he himself had explicated. But they did not believe that he was still alive, and did not show him honor.

Alexander Kirkpatrick notes an affinity of this psalm with Psalm 85, which begins LORD, thou hast been favourable unto thy land.