Lifnei iver (Hebrew: "before the blind") is one of the 613 mitzvot (commandments) in Jewish law. It is based on: "Before the blind, do not put a stumbling block" (Leviticus 19:14). It is one of the offenses spoken of in the Talmud as punishable by the Cherem.

Many halakhic principles are derived from this commandment. The oral law expands the ramifications of this law from its literal sense. The Sifra (a midrash from the time of the Mishna) states that, in its figurative sense, the commandment forbids one to give bad or damaging advice; after all, the receiving party is "blind" in this matter and will "stumble" as a result of the improper guidance (Sifra de-vei Rav, Kedoshim 2:14). It is therefore figurative for misleading anyone.

The Talmud (tractate Avodah Zarah 6b) expands the principle to include giving anyone the means or opportunity to commit a sin where that person would otherwise lack these. Examples given there include giving a cup of wine to a person who has taken the nazirite vow (which forbids him/her to partake in wine or grape products).

Lifnei iver as a principle recurs many times in practical Jewish law. The authoritative Shulkhan Arukh, for example, warns a father not to physically chastise older children, as this will only entice them to hit back, which is a capital offence (Shulkhan Arukh, Yorei Deah 240:20).