Skipjack tuna grows up to a length of 1m and they weigh around 18kg. Similar to other tunas, skipjack tuna has a spindle-shaped body and is mostly without scales. It has two dorsal fines, one with the spines and one without it. Several finlets follow the second dorsal fin. The anal fin is just beneath the second dorsal fin and is followed by finlets. The posterior peduncle consist of three keels; the larger one between the two smaller ones.
The skipjack tuna fish has a distinguishing dark lines extending towards the tail from behind the corselet back. They have a silvery lower flank and belly. It has a deep purplish blue back. Their teeth are conical in a single row.