Margaret Valdemarsdatter (Norwegian: Margrete Valdemarsdotter) (1353 - October 28, 1412) was Queen of Norway, Regent of Denmark and Sweden, and founder of the Kalmar Union which joined the Scandinavian countries for over a century.
She is known in Denmark as "Margrete I", to distinguish her from the current queen, but she never (except for a brief period in 1375) actually styled herself Queen of Denmark; rather she called herself "Margrete, by the grace of God, Valdemar Daneking's daughter" and "Denmark's rightful heir" when referring to her rulership in Denmark. Others simply referred to her as the "Lady Queen" without specifying what she was queen (or female king) of, but not so Pope Boniface IX, who blatantly styled her "Queen of Denmark" or "Queen of Denmark, Norway and Sweden" in his letters.
As to Norway, she was known as Queen (queen-consort, then dowager queen) and regent. And, as to ruling Sweden, she was Dowager Queen and Plenipotentiary Ruler. When she married Haakon, in 1363, he was yet co-king of Sweden (and Margaret thus was its queen), and despite of becoming deposed, they never relinquished the title (Haakon actually held Sweden's westernmost provinces throughout his reign up to his death). Therefore, when Swedes expelled Albert I in 1389, Margaret theoretically just resumed her original position.