William I of Württemberg
On 24 January 1816, in Saint Petersburg, he married his first cousin, Grand Duchess Catherine Pavlovna of Russia (1788–1819), daughter of Emperor Paul I of Russia (1754–1801) and Princess Sophie Dorothea of Württemberg (1759–1828). The couple had two children:
Sophia of Prussia
Sophia and Constantine's marriage was harmonious during the first years. However, faithfulness was not the greatest quality of the "Diadochos" and his wife soon had to deal with his numerous extramarital affairs. Initially shocked by what she saw as a betrayal, Sophia soon followed the example of her mother-in-law and condoned the behavior of her husband. From 1912, however, the couple became noticeably separated. At that time, Constantine began an affair with Countess Paola von Ostheim ("née" Wanda Paola Lottero), an Italian stage actress who had recently divorced from Prince Hermann of Saxe-Weimar-Eisenach; this relationship lasted until Constantine's death.