| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 641 | Junior | Latin | Younger. Used in the United States to distinguish a son from his father, when both bear the same name. | M | |
| 642 | Junius | Latin | Youthful. | M | |
| 643 | Junus | Latin | Youthful. | M | |
| 644 | Jupiter | Latin | Supreme god. | M | |
| 645 | Justain | Latin | Just; upright; righteous. Form of New Testament Biblical name Justus. | M | |
| 646 | Justice | Latin | Just; upright; righteous. Form of New Testament Biblical name Justus. | M | |
| 647 | Justin | Latin | Just. Righteous. Form of New Testament Biblical name Justus. Almost exclusively an Irish name until the 20th century. | M | |
| 648 | Justis | Latin | Just; upright; righteous. Form of New Testament Biblical name Justus. | M | |
| 649 | Justus | Latin | Just. | M | |
| 650 | Justyn | Latin | Just. | M | |
| 651 | Kasen | Latin | Helmeted. | M | |
| 652 | Kauldi | Latin | Lame. | M | |
| 653 | Kees | Latin | Horn. | M | |
| 654 | Kelman | Latin | Merciful. | M | |
| 655 | Kerstan | Latin | Christian. | M | |
| 656 | Kester | Latin | From the Roman camp. | M | |
| 657 | Kilian | Latin | Blind. | M | |
| 658 | Killian | Latin | Blind. | M | |
| 659 | Kinden | Latin | Born fifth. | M | |
| 660 | Kindin | Latin | Born fifth. | M | |