Objection 1. It would seem that penance is not a virtue. For penance is a sacrament numbered among the other sacraments, as was shown above (84, 1; 65, 1). Now no other sacrament is a virtue. Therefore neither is penance a virtue.
Objection 2. Further, according to the Philosopher (Ethic. iv, 9), "shame is not a virtue," both because it is a passion accompanied by a bodily alteration, and because it is not the disposition of a perfect thing, since it is about an evil act, so that it has no place in a virtuous man. Now, in like manner, penance is a passion accompanied by a bodily alteration, viz. tears, according to Gregory, who says (Hom. xxxiv in Evang.) that "penance consists in deploring past sins": moreover it is about evil deeds, viz. sins, which have no place in a virtuous man. Therefore penance is not a virtue.
Objection 3. Further, according to the Philosopher (Ethic. iv, 3), "no virtuous man is foolish." But it seems foolish to deplore what has been done in the past, since it cannot be otherwise, and yet this is what we understand by penance. Therefore penance is not a virtue.
On the contrary, The precepts of the Law are about acts of virtue, because "a lawgiver intends to make the citizens virtuous" (Ethic. ii, 1). But there is a precept about penance in the Divine law, according to Matthew 4:17: "Do penance," etc. Therefore penance is a virtue.
I answer that, As stated above (Objection 2; 84, 10, ad 4), to repent is to deplore something one has done. Now it has been stated above (84, 9) that sorrow or sadness is twofold. First, it denotes a passion of the sensitive appetite, and in this sense penance is not a virtue, but a passion. Secondly, it denotes an act of the will, and in this way it implies choice, and if this be right, it must, of necessity, be an act of virtue. For it is stated in Ethic. ii, 6 that virtue is a habit of choosing according to right reason. Now it belongs to right reason than one should grieve for a proper object of grief as one ought to grieve, and for an end for which one ought to grieve. And this is observed in the penance of which we are speaking now; since the penitent assumes a moderated grief for his past sins, with the intention of removing them. Hence it is evident that the penance of which we are speaking now, is either a virtue or the act of a virtue.
Reply to Objection 1. As stated above (84, 1, ad 1; 2,3), in the sacrament of Penance, human acts take the place of matter, which is not the case in Baptism and Confirmation. Wherefore, since virtue is a principle of an act, penance is either a virtue or accompanies a virtue, rather than Baptism or Confirmation.
Reply to Objection 2. Penance, considered as a passion, is not a virtue, as stated above, and it is thus that it is accompanied by a bodily alteration. On the other hand, it is a virtue, according as it includes a right choice on the part of the will; which, however, applies to penance rather than to shame. Because shame regards the evil deed as present, whereas penance regards the evil deed as past. Now it is contrary to the perfection of virtue that one should have an evil deed actually present, of which one ought to be ashamed; whereas it is not contrary to the perfection of virtue that we should have previously committed evil deeds, of which it behooves us to repent, since a man from being wicked becomes virtuous.
Reply to Objection 3. It would indeed be foolish to grieve for what has already been done, with the intention of trying to make it not done. But the penitent does not intend this: for his sorrow is displeasure or disapproval with regard to the past deed, with the intention of removing its result, viz. the anger of God and the debt of punishment: and this is not foolish.
Source: Summa Theologica (New Advent)