The Iambic trimeter, in classical Greek and Latin poetry, is a meter of poetry consisting of three iambic metra (each of two feet) per line. In English poetry, it refers to a meter with three iambic feet.

In ancient Greek poetry and Latin poetry, an iambic trimeter is a quantitative meter, in which a line consists of three iambic metra. Each metron consists of the pattern | x – u – |, where "–" represents a long syllable, "u" a short one, and "x" an anceps (either long or short). Resolution was common, especially in the first two metra of the line, so that any long or anceps syllable except the last could be replaced by two short syllables (see for example Euripides#Chronology), making a total of 13 or more syllables. It is the most common meter used for the spoken parts (as opposed to the sung parts) of Ancient Greek tragedy, comedy, and satyr plays. It is also common in iambus or 'blame poetry', although it is not the only meter for that genre.^[1]

In Latin, the iambic trimeter was adapted for the spoken parts of Roman plays, especially Roman comedy. The form used in Roman comedy is usually known as the iambic senarius. The iambic trimeter was also used in the Epodes of Horace, the fables of Phaedrus (fabulist), proverbs of Publilius Syrus, and tragedies of Seneca the Younger.

In the accentual-syllabic verse of English, German, and other languages, however, the iambic trimeter is a meter consisting of three iambs (disyllabic units with stress on the second syllable) per line, making a line of six syllables.

The iambic trimeter was imitated in Latin by 2nd century BC comic playwrights such as Plautus and Terence, where it is known as the iambic senarius. It is the most commonly used meter in their plays, especially in Terence, and it is the only meter which was used purely for dialogue without musical accompaniment. It was the preferred meter of the Roman fabulist Phaedrus in the first century AD. In Latin the basic meter was as follows:

| x – x – | x – x – | x – u – |

That is to say, the third and seventh elements, which were always short in Greek, were anceps (either long or short) in Latin; in fact they are long 60% of the time, while the Greek anceps syllables (the first, fifth, and ninth elements) are long in 80–90% of lines.^[2] As in the Greek trimeter, any long or anceps syllable except the last could be replaced with a double short syllable (u u). As in Greek, there was usually (though not always) a caesura (word-break) after the fifth element.

An example of a Latin iambic senarius (from the prologue to Plautus' Aulularia) is the following:

nē quís mīrétur quí sim, paúcīs éloquár.

| – – – – | – – – – | – – u – |

"In case anyone should wonder who I am, let me explain in a few words."

A difference between Latin and Greek iambics was that the Latin senarius was partly accentual, that is to say the words were arranged in such a way that very often (especially in the first half of the verse), the word accents coincided with the strong points of the line, that is the 2nd, 4th, 6th etc. elements of the verse. Thus even in lines where nearly all the syllables were long as in the above verse, it is possible to feel the iambic rhythm of the line.

Any long or anceps element except the last could be resolved into two short syllables, giving rise to lines like the following (the resolved elements are underlined):

nunc hódi(e) Amphítruō véniēt húc ab ěxércitú

| – uu – uu | – uu – – | uu – u – |

"Right now today Amphitryon will come here from the army"

The above line also illustrates another feature found in Plautus and Terence, namely iambic shortening or brevis breviāns, where the syllables ab ex- are scanned as two short syllables.

In English and similar accentual-syllabic metrical systems, a line of iambic trimeter consists of three iambic feet. The resulting six-syllable line is very short, and few poems are written entirely in this meter.

The 1948 poem "My Papa's Waltz" by Theodore Roethke uses the trimeter:

...We romped until the pans

Slid from the kitchen shelf;

My mother's countenance

Could not unfrown itself.

William Blake's "Song ('I Love the Jocund Dance')" (1783) uses a loose iambic trimeter that sometimes incorporates additional weak syllables:

I love the jocund dance,

The softly breathing song,

Where innocent eyes do glance,

And where lisps the maiden's tongue.

I love the laughing gale,

I love the echoing hill,

Where mirth does never fail,

And the jolly swain laughs his fill.