Garegin or Karekin Pastermadjian, better known by his nom de guerre Armen Garo or Armen Karo (9 February 1872 – 23 March 1923) was an Armenian nationalist activist and politician. He was one of the masterminds of the 1896 occupation of the Ottoman Bank in response to the Hamidian massacres, in which hundreds of thousands of Armenians were murdered, and Operation Nemesis, in which several perpetrators of the Armenian genocide were assassinated. Between 1918 and 1920 he served as the first ambassador to the United States from the First Republic of Armenia.

Karekin Pastermadjian was born in Karin (present day Erzurum Province). He finished his elementary education as one of the first graduates of the Sanasarian College of Erzerum (Sanasarian Varjaran Academy) in 1891. Later in 1894, he continued his studies in France to study agriculture at the Agricultural School of Nancy-Université.

His plans to return to his hometown after graduating came to a halt when massacres and conflict began in Zeitun. He left his studies to aid his compatriots in Zeitun. He soon found himself in Geneva, where he became an activist alongside his friends. He was sent to Egypt to assist the Zeitun Resistance. With Sarkis Srentz, Haig Tiriakian (who assumed the name Hratch), and Max Zevrouz, he left Egypt and returned to the Ottoman Empire to participate in the efforts of the ARF. It was around this time that Karekin Pastermadjian first assumed the name Armen Garo.

The 1896 Ottoman Bank Takeover, executed on 26 August 1896, was his and Papken Siuni's brainchild. The attack took place at about 1 p.m. Armen Garo entered 10 minutes prior to the attack. He was to keep the teller busy and the bank officers from escaping. His men didn't arrive quickly so he went to a coffee shop across the street. When he saw the men through the doorway, he ran out to meet them. During the long and bloody battle, the group leader, Papken Siuni, was killed. Armen Garo took over as leader, ordering his group for much of the standoff.

After these events, Karekin Pastermadjian returned to Europe to continue his unfinished studies. French Foreign Minister Gabriel Hanotaux declared the Armenians who had been connected with the Ottoman Bank Takeover as persona non grata and denied their stay in France. Armen Garo transferred to Switzerland and studied natural sciences at the University of Geneva.

During his studies, he continued his activities with the Armenian Revolutionary Federation. He was on the delegate roster of the second ARF general conference in 1898, representing the party committee in Egypt. Despite his youth, he had gained trust and authority in the leading circles of the Armenian Revolutionary Federation. In 1900 he completed his courses at the University of Geneva and received a doctoral degree in physical chemistry. In 1901 he founded a lab in Tiflis for chemical research.

When the 1915 Armenian genocide broke out, Armen Garo became very depressed and sick. When Armenia lost its independence, his sickness grew worse. He died of heart disease in Geneva, Switzerland on 23 March 1923, where he was attending a conference on Russia.

He is regarded as a national hero by Armenians, especially ARF supporters due to his numerous efforts in helping his people.