The fruit is a dark brown, oblong, flattened achene. There is a pappus of white hairs present at one end of the fruit.
A perennial plant with sticky, hairy, greyish-green, untoothed leaves. The leaves are also narrow with pointed lobes. The auricles of the leaves are pointed and clasp the stems. Marsh Sow-thistle is similar in appearance to Perennial Sow-thistle (Sonchus arvensis). Marsh Sow-thistle is taller with less regularly serrated leaf margins. The leaves also end in a long, triangular point and have a waxy coating.