Carl August Rathjens (11 March 1887 – 29 July 1966) was a German geographer whose primary interests were in South Arabian historiography, geology and ethnography. He made several visits to Yemen, in the years 1927, 1931, 1934 and 1938. He is considered the greatest scholar of Yemeni research in the 20th century. He contributed more than any other in conducting scientific and ethnographic research, resulting in a wide range of findings, and he has left over 2500 ethnographical items and some 4000 positive and negative photographs from South Arabia.

Born the son of a teacher, Carl Rathjens began his academic studies in 1906 in the University of Hamburg, and then continued to expand his higher education in the universities of Kiel, Berlin and Munich on the subjects of geography, geology, cartography, meteorology, astronomy, botany, zoology, demography, sociology and economy.

Rathjens travelled to Egypt as a young German student of geography, geology, astronomy, meteorology and biology. At short notice, and without planning, he continued on his journey and traveled to Ethiopia in 1908, accepting a friend's proposal to visit his uncle who officiated there as a priest. During his stay in that country, he met Jews in the Tigré region of Abyssinia and studied their history, religion and culture.

In 1911 he earned his doctorate under Erich von Drygalski with the thesis, Beiträge zur Landeskunde von Abessininen ("Contributions to the Geography of Abyssinia"), in which he proposed to his professor the writing of a follow-up thesis for a habilitation degree, entitled, Die Juden in Abessinien, which would permit him to instruct as a professor. His study on the Jews of Ethiopia was published in 1921. After a short period at the State Zoological Institute in Munich, Rathjens worked from 1911 to 1921 at the Hamburg Colonial Institute and then worked for the World Economic Archives (Welt Wirtschaftsarchive), until his dismissal on political grounds in 1933, for refusal to join the Nazi party. During his years with the World Economic Archives, he would also lecture in the Geographical Department at the University of Hamburg.

In 1927, Carl Rathjens, Hermann von Wissmann and an orientalist by the name of Erika Apitz travelled to Jeddah, Saudi Arabia, with an aim to make a geological survey of that country and to document the fauna and flora in regions between Jeddah and Mecca. At Jeddah, Daniel van der Meulen, who was a Dutch diplomat and Consul at Jeddah, invited them to stay in his house while waiting for the visa from King Ibn Saud. The king's answer reached them after ten days, in which he explicitly forbade them from entering the interior of his country. The three scholars were disappointed and frustrated. They left Jeddah on a ship which brought them to East Africa. There, they felt uncomfortable with the idea about returning at that time to Europe, having not fulfilled their mission. Therefore, they decided to visit South Arabia, which was not originally a part of their itinerary.

Rathjens, with his two compatriots, disembarked from a boat which brought them to Yemen the first time in 1927. Upon their arrival in Hodeida, Carl Rathjens wrote to his family in Hamburg that when he entered Yemen he had “left civilization behind him; there are no banks, neither hotels, nor embassies. There are no cars, nor asphalt roads. There isn't any post office in Hodeida; one can use the Telegraph only in Hodeida and Sana’a. There are only two stamps in use in the country and they are not recognized elsewhere outside of Yemen. In the houses there is no tap water; neither is there electricity. The families hardly ever used furniture in their houses. The industrial revolution had not reached this country. Most of the products were hand-made; neither machines, nor technological utensils were used.” Compared with Europe, he felt as though he had returned in time to the Middle-Ages. Yemen was, for him, like jumping back five-hundred years in a time machine.

Already on his first visit, he developed a good rapport with the king, the Imām Yaḥyā Ḥamīd ad-Dīn (1864-1948) and his five elder sons, all of whom were serving the country and had ministerial posts, or else managed an important position in the court. Rathjens also nurtured a good relationship with the Prime Minister, Abdallah al Amri. With him he communicated in English. He had a good rapport with the Minister of Foreign Affairs, Muhammad Ragib, with whom he communicated in French. Rathjens and his colleagues were the official guests of the Imam Yahya. They were housed in the state guest house in Bir el Azab, near the Jewish Quarter at Sana'a. In his letters to family members and friends, he had repeatedly described how he enjoyed the hospitality of the king and felt that his life in Sana'a was like a chapter taken from the book, A Thousand and One Nights.

Rathjens and his two colleagues were accredited with making the first archaeological dig in Yemen. The Imam Yahya had sent the three German scholars to Huqqah to dig an ancient grave. Rathjens could not convince the Imam that the three of them were not archaeologists. The three German scholars left for Huqqah near Haggah with a delegation of fifty soldiers and an inspector. With the help of fifty local workers, they unearthed a grave from the second century CE. Rathjens assumed that in Yemen, archaeology was in its nascent stages and that it should be encouraged and expanded. He reasoned, however, that the research should be done in a professional way, employing scientific methods. This gave him the incentive to request from Imam Yahya that he establish a Ministry of Antiquities and to build a museum in which the archaeological findings could be stored and preserved. Rathjens also suggested that the Imam should invite from Germany a team consisting of an archaeologist and philologist for ancient Semitic languages in order to inspect the excavations, as well as to document their findings and do the deciphering of the inscriptions, by using strict scientific methods as those used in Europe.