Objection 1. It seems that boasting is a mortal sin. For it is written (Proverbs 28:25): "He that boasteth, and puffeth himself, stirreth up quarrels." Now it is a mortal sin to stir up quarrels, since God hates those that sow discord, according to Proverbs 6:19. Therefore boasting is a mortal sin.
Objection 2. Further, whatever is forbidden in God's law is a mortal sin. Now a gloss on Sirach 6:2, "Extol not thyself in the thoughts of thy soul," says: "This is a prohibition of boasting and pride." Therefore boasting is a mortal sin.
Objection 3. Further, boasting is a kind of lie. But it is neither an officious nor a jocose lie. This is evident from the end of lying; for according to the Philosopher (Ethic. iv, 7), "the boaster pretends to something greater than he is, sometimes for no further purpose, sometimes for the sake of glory or honor, sometimes for the sake of money." Thus it is evident that it is neither an officious nor a jocose lie, and consequently it must be a mischievous lie. Therefore seemingly it is always a mortal sin.
On the contrary, Boasting arises from vainglory, according to Gregory (Moral. xxxi, 17). Now vainglory is not always a mortal sin, but is sometimes a venial sin which only the very perfect avoid. For Gregory says (Moral. viii, 30) that "it belongs to the very perfect, by outward deeds so to seek the glory of their author, that they are not inwardly uplifted by the praise awarded them." Therefore boasting is not always a mortal sin.
I answer that, As stated above (Question 110, Article 4), a mortal sin is one that is contrary to charity. Accordingly boasting may be considered in two ways. First, in itself, as a lie, and thus it is sometimes a mortal, and sometimes a venial sin. It will be a mortal sin when a man boasts of that which is contrary to God's glory--thus it is said in the person of the king of Tyre (Ezekiel 28:2): "Thy heart is lifted up, and thou hast said: I am God"--or contrary to the love of our neighbor, as when a man while boasting of himself breaks out into invectives against others, as told of the Pharisee who said (Luke 18:11): "I am not as the rest of men, extortioners, unjust, adulterers, as also is this publican." Sometimes it is a venial sin, when, to wit, a man boasts of things that are against neither God nor his neighbor. Secondly, it may be considered with regard to its cause, namely, pride, or the desire of gain or of vainglory: and then if it proceeds from pride or from such vainglory as is a mortal sin, then the boasting will also be a mortal sin: otherwise it will be a venial sin. Sometimes, however, a man breaks out into boasting through desire of gain, and for this very reason he would seem to be aiming at the deception and injury of his neighbor: wherefore boasting of this kind is more likely to be a mortal sin. Hence the Philosopher says (Ethic. iv, 7) that "a man who boasts for the sake of gain, is viler than one who boasts for the sake of glory or honor." Yet it is not always a mortal sin because the gain may be such as not to injure another man.
Reply to Objection 1. To boast in order to stir quarrels is a mortal sin. But it happens sometimes that boasts are the cause of quarrels, not intentionally but accidentally: and consequently boasting will not be a mortal sin on that account.
Reply to Objection 2. This gloss speaks of boasting as arising from pride that is a mortal sin.
Reply to Objection 3. Boasting does not always involve a mischievous lie, but only where it is contrary to the love of God or our neighbor, either in itself or in its cause. That a man boast, through mere pleasure in boasting, is an inane thing to do, as the Philosopher remarks (Ethic. iv, 7): wherefore it amounts to a jocose lie. Unless perchance he were to prefer this to the love of God, so as to contemn God's commandments for the sake of boasting: for then it would be against the charity of God, in Whom alone ought our mind to rest as in its last end.
To boast for the sake of glory or gain seen to involve an officious lie: provided it be do without injury to others, for then it would once become a mischievous lie.
Source: Summa Theologica (New Advent)