Fellatio (also known as fellation, and in slang as blowjob, BJ, giving head, or sucking off) is an oral sex act consisting of the stimulation of a penis by using the mouth. Oral stimulation of the scrotum may also be termed fellatio, or colloquially as teabagging.

It may be performed by a sexual partner as foreplay before other sexual activities, such as vaginal or anal intercourse, or as an erotic and physically intimate act of its own. Fellatio creates a risk of contracting sexually transmitted infections (STIs), but the risk is significantly lower than that of vaginal or anal sex, especially for HIV transmission.

Most countries do not have laws banning the practice of fellatio, though some cultures may consider it taboo. People may also refrain from engaging in fellatio due to personal preference, negative feelings, or sexual inhibitions. Commonly, people do not view oral sex as affecting the virginity of either partner, though opinions on the matter vary.

The English noun fellatio comes from the Latin fellātus, the past participle of the verb fellāre, meaning "to suck". In fellatio, the -us is replaced by the -io while the declension stem ends in -ion-, which gives the suffix the form -ion (cf. French fellation). The -io(n) ending is used in English to create nouns from Latin adjectives and it can indicate a state or action wherein the Latin verb is being, or has been, performed.

Further English words have been created based on the same Latin root. A person who performs fellatio upon another (i.e. who fellates) may be termed a fellator. Latin's gender based declension means this word may be restricted to describing a male. The equivalent term for a female is fellatrix.

The essential aspect of fellatio is oral stimulation of the penis (including the shaft and glans) through sucking with the mouth, use of the tongue for licking, using the lips, or some combination. One method is the sex partner taking the penis into the mouth and moving smoothly up and down to a rhythm while being careful to avoid contact with the teeth. Fellatio also includes oral stimulation of the scrotum, whether licking, sucking or taking the entire scrotum into the mouth. During the act, orgasm may be achieved and semen may be ejaculated into the partner's mouth. When the penis is thrust into someone's mouth, it is called irrumatio, though the term is rarely used; "face-fucking" is a more prevalent colloquial term.

Performing fellatio can trigger the gag reflex.

It is physically possible for men with sufficient flexibility, penis size, or both, to perform fellatio on themselves as a form of masturbation, in an act called autofellatio. However, few men possess the flexibility and penis length to safely perform the necessary frontbend.