The 218th mitzvah is that a rapist is commanded to marry his victim.1

The source of this commandment is G‑d's statement2 (exalted be He), "Because he violated her, he must then take her to be his wife, and he may not send her away as long as he lives."

It is explained in tractate Makkos3 that the prohibition on the rapist not to divorce her, i.e., "he may not send her away," is considered "a prohibition which is preceded by a positive commandment." Our Sages stated there,4 "Why [should a rapist who marries his victim, and then divorces her, and then marries her again not receive lashes]? This is a prohibition which is preceded by a positive commandment?!" This statement demonstrates that G‑d's statement, "He must then take her as his wife," counts as a positive commandment.

The details of this mitzvah are explained in the third and fourth chapters of Kesubos.