Pyrrhosoma nymphula can reach a body length of 33–36 millimetres (1.3–1.4 in).
These large and robust damselflies show black legs and wing spots in both sexes.

Mature males have red eyes and a black thorax with red shoulder stripes in mature specimen, but yellow stripes in immature.
In fact ante-humeral stripes change to red with age.
Abdomen is red with black small rings and bronze-black bands towards the apex (segments 7-9).
Wings are hyaline, with a blackish pterostigma.
Mature females occur in three colour forms (typical, fulvipes and melanotum), from mostly black to mostly red, but all have yellow bands around the abdominal segments.
Some intermediate forms also exist.

The form typica has more black on its abdominal segments than the form fulvipes, particularly on segment 6.
Immatures have lighter eyes and have yellow stripes on the thorax, not red.
In the form melanotum females show the upper surface of the abdomen almost entirely black.

These damselflies can easily be confused with small red damselflies, but the latter has orange legs, while the large red damselfly has black legs.
In Greece and Albania a closely related species occurs, the Greek red damsel (Pyrrhosoma elisabethae).
They look very much the same, the females only having a slightly different pronotum with deep folds in the hind margin.
The males differ in their lower appendages, which are longer than the upper ones, while the black hook on the lower appendages is half as long as in the large red damselfly.
The appendages of the large red damselfly can be seen in the gallery below.
Pyrrhosoma nymphula can reach a body length of 33–36 millimetres (1.3–1.4 in). These large and robust damselflies show black legs and wing spots in both sexes.
Mature males have red eyes and a black thorax with red shoulder stripes in mature specimen, but yellow stripes in immature. In fact ante-humeral stripes change to red with age. Abdomen is red with black small rings and bronze-black bands towards the apex (segments 7-9). Wings are hyaline, with a blackish pterostigma. Mature females occur in three colour forms (typical, fulvipes and melanotum), from mostly black to mostly red, but all have yellow bands around the abdominal segments. Some intermediate forms also exist.
The form typica has more black on its abdominal segments than the form fulvipes, particularly on segment 6. Immatures have lighter eyes and have yellow stripes on the thorax, not red. In the form melanotum females show the upper surface of the abdomen almost entirely black.
These damselflies can easily be confused with small red damselflies, but the latter has orange legs, while the large red damselfly has black legs. In Greece and Albania a closely related species occurs, the Greek red damsel (Pyrrhosoma elisabethae). They look very much the same, the females only having a slightly different pronotum with deep folds in the hind margin. The males differ in their lower appendages, which are longer than the upper ones, while the black hook on the lower appendages is half as long as in the large red damselfly. The appendages of the large red damselfly can be seen in the gallery below.
Large red damselfly