The ruddy darter reaches a body length of 34 to 36 millimeters and a wingspan of up to 6 cm. It is smaller than the common darter. The head, thorax and abdomen of the mature male are vivid red. The female is slightly smaller and has a golden-yellow color with black markings.
The ruddy darter has entirely black legs. This distinguishes it from the otherwise very similar common darter (Sympetrum striolatum) and vagrant darter (Sympetrum vulgatum), both of which show yellow stripes on their legs.
The ruddy darter favors shallow water with emergent vegetation. Both sexes usually perch on raised vantage points from which they can scan for prey. They will usually return to the same perch after chasing prey or banishing a rival.