Teuthowenia megalops are moderately sized glass squid.
They are sexually dimorphic, reaching mantle lengths between 37 and 40 cm (15 and 16 in) for females, and 18.2 to 24.4 cm (7.2 to 9.6 in) for males.
They have very large forward-facing eyes, each possessing three bioluminescent organs (photophores), a defining characteristic of the genus Teuthowenia.
One of the photophores is oval-shaped and small, located at the upper part near the lens of each eye.
The other two are concentric and located at the sides and bottom of the eyeball.
One of them is very narrow and curves halfway around the lens.;
the other is very large and crescent-shaped, straddling half of the bottom surface of the eyeball.

The mantle is conical and elongated with a narrow rear tip.
The mantle walls are thin and leathery.
The funnel locking-apparatus are small and oval to spindle-shaped.
A single cartilaginous tubercle is present at the mantle margins, though rarely it may be absent.
The fins are long and narrow, starting at about 40 to 60% of the length of the mantle, far from where the body is the widest.
They extend past the rear tip of the body, fuse with each other at the middle, and end with a pair of small lobes.

The arms are short to medium length, about a fourth to half of the mantle length.
In subadults and females, the third pair of arms is the longest, followed by the second, first, and the fourth pairs (length formula: III>II>I>IV).
Sometimes the fourth and first pairs may be of equal or almost equal length (length formula: III>II>I=IV).
Adult males may have the first and second pair of equal length (but shorter than the third pair), or the first pair may be the longest of all the arms (length formula: III>I=II>IV or I>III>II>IV).
They all possess two rows of cup-shaped to globular suckers that lack hooks.
The suckers on the midportion of the third and second pair of arms are greatly enlarged, exceeding the width of the arms.

In subadults of both sexes and mature females, the basal and mid-arm suckers are usually smooth, though the small terminal suckers possess six to ten short triangular teeth on the distal and lateral rims.
About 6 to 9% of the tips of all the arms of subadult and adult females are also modified into grasping end-organs.
In adult males, the arms are all sexually modified in some way.
The first pair becomes elongated and thicker, with swollen midportions and expanded protective membranes.
The second pair has elongated and swollen tips, with reduced protective membranes.
The third and fourth pairs all have slender whip-like tips.
The suckers on the tips of the second pair have swollen collars and possess small pointed teeth around their entire margin, while the rest of the suckers on all the arms have small rounded or triangular teeth on the distal and lateral rims.
The mid-arm and basal suckers of the third and fourth pairs also have swollen collars and greatly reduced openings.

The tentacles are short and muscular.
They have four rows of carpal ("wrist") suckers, arranged into a zigzag pattern on ½ to ⅔ of the distal end of the tentacles leading towards the tentacular clubs.
The suckers on the tentacular club are set on long protrusions and possess numerous short, sharp teeth on their entire margins.
The largest of these possess 19 to 24 teeth.
Teuthowenia megalops are moderately sized glass squid. They are sexually dimorphic, reaching mantle lengths between 37 and 40 cm (15 and 16 in) for females, and 18.2 to 24.4 cm (7.2 to 9.6 in) for males. They have very large forward-facing eyes, each possessing three bioluminescent organs (photophores), a defining characteristic of the genus Teuthowenia. One of the photophores is oval-shaped and small, located at the upper part near the lens of each eye. The other two are concentric and located at the sides and bottom of the eyeball. One of them is very narrow and curves halfway around the lens.; the other is very large and crescent-shaped, straddling half of the bottom surface of the eyeball.
The mantle is conical and elongated with a narrow rear tip. The mantle walls are thin and leathery. The funnel locking-apparatus are small and oval to spindle-shaped. A single cartilaginous tubercle is present at the mantle margins, though rarely it may be absent. The fins are long and narrow, starting at about 40 to 60% of the length of the mantle, far from where the body is the widest. They extend past the rear tip of the body, fuse with each other at the middle, and end with a pair of small lobes.
The arms are short to medium length, about a fourth to half of the mantle length. In subadults and females, the third pair of arms is the longest, followed by the second, first, and the fourth pairs (length formula: III>II>I>IV). Sometimes the fourth and first pairs may be of equal or almost equal length (length formula: III>II>I=IV). Adult males may have the first and second pair of equal length (but shorter than the third pair), or the first pair may be the longest of all the arms (length formula: III>I=II>IV or I>III>II>IV). They all possess two rows of cup-shaped to globular suckers that lack hooks. The suckers on the midportion of the third and second pair of arms are greatly enlarged, exceeding the width of the arms.
In subadults of both sexes and mature females, the basal and mid-arm suckers are usually smooth, though the small terminal suckers possess six to ten short triangular teeth on the distal and lateral rims. About 6 to 9% of the tips of all the arms of subadult and adult females are also modified into grasping end-organs. In adult males, the arms are all sexually modified in some way. The first pair becomes elongated and thicker, with swollen midportions and expanded protective membranes. The second pair has elongated and swollen tips, with reduced protective membranes. The third and fourth pairs all have slender whip-like tips. The suckers on the tips of the second pair have swollen collars and possess small pointed teeth around their entire margin, while the rest of the suckers on all the arms have small rounded or triangular teeth on the distal and lateral rims. The mid-arm and basal suckers of the third and fourth pairs also have swollen collars and greatly reduced openings.
The tentacles are short and muscular. They have four rows of carpal ("wrist") suckers, arranged into a zigzag pattern on ½ to ⅔ of the distal end of the tentacles leading towards the tentacular clubs. The suckers on the tentacular club are set on long protrusions and possess numerous short, sharp teeth on their entire margins. The largest of these possess 19 to 24 teeth.
Teuthowenia megalops