H. medicinalis breeds once during an annual season that spans June through August. It also remains fertile over a period of years,unlike most other leech species. The act of copulation takes place on land, where one leech attaches ventrally to one another by means of a mucus secretion. All leeches are hermaphroditic and fertilization is internal. Sperm is injected into the vagina by an extendable copulatory organ. A cocoon is formed around the clitellum and slips off the anterior section of the leech. The whole egg sac is laid in damp soil usually just above the shoreline. After about 14 days, the eggs hatch as fully formed miniature adults.


The medicinal leech is amphibious, needing both land and water, and resides exclusively in fresh water. A typical habitat for H. medicinalis would be a small pond with a muddy bottom edged with reeds and in which frogs are at least seasonally abundant


The medicinal leech has a cylindrical, dorsoventrally flattened body divided into thirty-three or thirty-four segments. The dorsal side is dark brown to black, bearing six longitudinal, reddish or brown stripes, and the ventral surface is speckled. All members bear a posterior and anterior disk-shaped sucker. The anterior sucker surrounds the oral opening where the teeth for incison are located. In addition, the medicinal leech has five pairs of eyes located on its front end. H. medicinalis has several pairs of testes and one pair of ovaries as well as a thickening of the body ring, known as a clitellum, which is visible during the breeding season.