Psalm 151 is the name given to a short psalm that is found in most copies of the Septuagint but not in the Masoretic text of the Hebrew Bible. The title given to this psalm in the Septuagint indicates that it is supernumerary, and no number is affixed to it: "This Psalm is ascribed to David and is outside the number. When he slew Goliath in single combat". It is included also in some manuscripts of the Peshitta.
Although for many years scholars believed that Psalm 151 might have been an original Greek composition and that “there is no evidence that Psalm 151 ever existed in Hebrew”, we now know from the Dead Sea scrolls that this psalm did in fact exist in Hebrew and was actually a part of the psalter used by the Qumran community.
Psalm 151 appears along with a number of canonical and non-canonical psalms in the Dead Sea scroll 11QPs(a) (named also 11Q5), a first century CE scroll discovered in 1956. The editio princeps of this manuscript was first published in 1963 by J. A. Sanders. This scroll contains two short Hebrew psalms which scholars now agree served as the basis for Psalm 151.
One of these Hebrew psalms, known as “Psalm 151a”, is reflected in verses 1–5 of the Greek Psalm 151, while verses 6 onward are derived from the other Hebrew psalm, known as “Psalm 151b” (which is only partially preserved). The composer has brought the two Hebrew psalms together in a manner that significantly changes their meaning and structure, but the influence of the Hebrew originals is still readily apparent. In some ways the Greek version of Psalm 151 does not seem to make good sense, and the Hebrew text provides a basis for a better understanding of what transpired in the creation of the Greek version. In comparison to the Hebrew text Sanders regards the Greek text of this psalm to be in places “desiccated”, “meaningless”, “truncated”, “ridiculous”, “absurd”, “jumbled”, and “disappointingly different”, all this the result of its having been “made from a truncated amalgamation of the two Hebrew psalms”. On details of translation, structure, and meaning of this psalm see especially the works of Skehan, Brownlee, Carmignac, Strugnell, Rabinowitz, Dupont-Sommer, and Flint.
The title of the psalm claims that it was written by King David after his battle with Goliath.
As it stands in the Greek text this apocryphal psalm celebrates how David was the least of his brothers, and yet God chose him to be anointed king (vv. 1-5). It goes on to commemorate how David cut off Goliath's head with the Philistine's own sword, and thereby removed Israel's disgrace (vv. 6-7).
The psalm assumes familiarity with and draws ideas and phraseology from certain portions of biblical material.