Plural of “series”
The plural of series is series.
Why
"Series" comes directly from Latin "series" (a row, chain, or succession), and it's one of a small class of Latin loanwords that entered English already ending in -s and simply never developed a distinct plural form — the singular and plural are spelled and pronounced identically.
This zero-plural behavior for "series" isn't the same historical mechanism as "sheep" or "deer" (ancient Old English neuter nouns); it's a separate, later phenomenon affecting certain Latin -s nouns that entered English through scholarly or scientific borrowing and simply kept their original Latin ending without adding an English plural marker on top of it. Publishing and streaming industries rely on "series" constantly to describe ongoing, multi-installment content, and because the word's plural and singular forms are identical, marketing copy sometimes has to add explicit numerals or qualifiers ("three hit series") specifically to avoid ambiguity that the noun itself doesn't resolve.
Usage note
Context and the surrounding verb are the only signals of number: "the series is popular" (one show) versus "several series are airing this fall" (multiple shows) — the noun itself never changes form regardless of which meaning is intended.
"Series" also functions as a mathematical term (a sequence of numbers following a pattern, as in an "arithmetic series"), and even in that specialized statistical and mathematical sense, the word behaves identically as a zero-plural noun, showing the pattern is fully consistent across every meaning "series" carries. Statisticians and economists also use "time series," a specific technical term for data collected sequentially over time, and that compound behaves exactly like the base noun, staying unchanged in both singular and plural forms regardless of how many separate time series are being analyzed together. Academic citation styles sometimes require noting whether a cited work is part of a larger series, using the identical unchanging noun regardless of how many volumes the series eventually includes.
Common real mistakes
Writing "serieses" as a plural is a genuine, if fairly rare, overregularization error; the correct plural is always identical to the singular, with number conveyed entirely through context, articles, and verb agreement rather than through any change to the noun itself. A further subtlety some writers miss: "series" pairs with a singular verb when referring to one show ("the series is excellent") and a plural verb when referring to more than one ("the series are all excellent"), meaning the verb, not the noun, carries the entire burden of signaling number in a sentence.
The wider pattern
"Series" joins a small set of Latin-derived English nouns — "species" among them, and "corps" in a related sense — that never picked up a separate plural form at all; whether one or several is meant gets signaled purely by surrounding context, with the word itself staying frozen.
Examples
- The streaming service released three new series this month.
- Her favorite series just finished its final season.
- Both series were filmed in the same city, years apart.
- The library carries several series aimed at young readers.
- A new documentary series explores climate change across five continents.
- The network canceled two series after disappointing ratings.
- This series has won several major awards over its run.
- Fans eagerly awaited the next installment in the series.
- The publisher launched three separate series in a single year.
- Multiple series share the same fictional universe and characters.
- The anthology series changes its entire cast and setting each season.
- Two competing series debuted the same week and split the audience.
- The publisher committed to releasing the series in six annual installments.
- A limited series differs from an ongoing show by having a fixed number of episodes planned from the start.
- The network greenlit two more series after the pilot tested well.
- Critics called it one of the strongest debut series in years.