Psalm 21, as The Nelson Study Bible notes, "is another of the royal psalms of David. Psalm 20 is a prayer of the king for God's blessing on his army. Psalm 21 is an assurance of God's blessing on the king [himself]. Both psalms, as is the case with all the royal psalms, speak ultimately of the great King to come, the Lord Jesus." Some see this psalm as one of thanksgiving after the victory in the battle referred to in the previous psalm.

David praises God for the blessings given to him as king. The king depended on God's strength: "How great is his joy in the victories You give!" (verse 1b, NIV). Here the NKJV has, "And in Your salvation how greatly shall he rejoice!" "One meaning of the Hebrew word salvation is 'room to breathe.' God had given King David a release from the pressures and constraints that bound him" (note on verse 1). Besides the joy of temporary deliverance from physical conditions, the verse also looks forward to ultimate spiritual salvation.

David enumerates the good acts of God on his behalf: kingship (a crown of pure gold), victory, desire of his heart, long life (forever and ever through salvation), deliverance, honor and majesty, blessings, God's presence and unfailing love (verses 2-7).

David asserts that his enemies are also God's enemies because "they intended evil against You," (verse 11) and they "hate You" (verse 8). David trusts God to deal with them (verse 9) "in the time of Your anger." Looking beyond David's own lifetime, God's anger will be evidenced to all during the Day of the Lord, a time of punishment prophesied throughout the Scriptures.