O sing unto the Lord a new song - See the notes at Psalms 33:3. This is the only addition made to the original form of the psalm. The word new here implies that there was some fresh occasion for celebrating the praises of God; that some event had occurred, or that some truth relating to the divine character had now been made known, which could not well be expressed in any psalm or hymn then in use. It is a call on all to celebrate the praises of the Lord in a “new” song - new, particularly, as it calls on “all the earth” to join in it; and possibly this was designed to suggest the idea that while that temple stood, a dispensation would commence, under which the distinction between the Jews and the Gentiles would be broken down, and all mankind would unite in the praise of God.
Sing unto the Lord, all the earth - All nations. All people had occasion to bless his name; to praise him. What he had done, what he was still doing, was of interest to all lands, and made an appeal to all people to praise him. The psalm is constructed on this supposition, that the occasion for praise referred to was one in which all people were interested; or, in other words, that Yahweh was the true God over all the nations, and that all people should acknowledge him.